Did Jesus exist before his life, and if not could the Trinity be added to?

God the Father said:

KJV Hebrews 1:8-12 But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom. Thou hast loved righteousness, and hated iniquity; therefore God, even thy God, hath anointed thee with the oil of gladness above thy fellows. And, Thou, Lord, in the beginning hast laid the foundation of the earth; and the heavens are the works of thine hands: They shall perish; but thou remainest; and they all shall wax old as doth a garment; And as a vesture shalt thou fold them up, and they shall be changed: but thou art the same, and thy years shall not fail.

Father testified to the preexistence of Christ. He has declared that Jesus is the Creator of heavens and earth.

Christ talking about his incarnation says in Psalms through David:

KJV Psalms 40:7-8 Then said I, Lo, I come: in the volume of the book it is written of me, I delight to do thy will, O my God: yea, thy law is within my heart.

Jesus said, concerning his preexistence during the patriarchal age:

John 8:56-58 Your father Abraham rejoiced to see my day: and he saw it, and was glad. Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham? Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.

'Before' in the verse above is definitely in the context of time and not place or preeminence, as the question of the Jews clarifies it in verse 57.

Jesus said again:

KJV John 5:37 And the Father himself, which hath sent me, hath borne witness of me. Ye have neither heard his voice at any time, nor seen his shape.

Jesus was sent. This presupposes the preexistence of the one being sent. One can't send something which does not exist.

John the beloved witnessed:

KJV John 1:18 No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him.

Jesus having been in His Father's bosom, having seen the Father is able to declare regarding Him.

Jesus said in Revelation:

KJV Revelation 22:12-13 And, behold, I come quickly; and my reward is with me, to give every man according as his work shall be. I am Alpha and Omega, the beginning and the end, the first and the last.

Jesus is the beginning and the end - the very words that the Father used in the first chapter of Revelation to qualify Himself.

Thus the preexistence of Christ is a theme that is woven across the Bible - both the Old and the New.

The logic why there has been and will forever be only three persons in the Godhead (called the Trinity) is very simple. This is how Christianity explains it:

1. Before creation, the Trinitarian Godhead already existed outside time in 3 Persons:

   • God the Father
   • God the Son (also called the Word; more precisely, internal divine procession by way of the intellect as an act of generation, c.f. John 8:42 "I proceeded from God")
   • God the Holy Spirit (who is the Love between them³; more precisely, internal divine spiration by way of the will, c.f. John 15:26 "the Spirit of truth, which proceedeth from the Father"). In fact, St. Thomas Aquinas taught that Love is the proper name of the Holy Spirit (S.T. Ia. Q37).

2. When God created the universe, time is created with it. Each moment in time is present to God. But to our point of view, revelation is progressive within time (history).

3. God the Father revealed Himself first ¹, including to Abraham (around 2,000 BC) and to Moses (around 1500 BC).

4. Then God the Son revealed Himself in the womb of virgin Mary (around 4 BC). He was born as baby Jesus who grew up as a regular human being YET retaining his divine nature as God the Son. So yes, he already existed before being conceived in Mary's womb as the Word, as one person of the Trinity. This incarnation works by God the Son adding on a human nature without eliminating any of his pre-existing divine nature.

5. After Jesus ascended to heaven, the Holy Spirit made his public appearance on Pentecost (10 days after ascension). The Holy Spirit is God giving Himself to the church as Love by external procession. This Third Person of the Godhead dwells within the heart of every believer so each believer can participate in the divine Love (the immanent / internal procession) who is between God the Father and God the Son.

As you can see, in proper Trinitarian understanding, especially the relationship between the three Persons of the Godhead, there is no more Person to be revealed. Instead of "trinity being added to God", the right way to understand this is that the Second Person of the Trinitarian Godhead adds a human nature.

As for your other question "Is that the limit and end of possible manifestations, or aspects or what the parts of the trinity are treated as? Or is it reasonable to conclude in the huge cosmos with vast timescales we now know about, that the trinity might need to be added to, as circumstances develop?", from the above explanation of how Christians understand the relation between God and creation, the answer is NO because the revelation is complete. We are awaiting the day of judgment, which can come at a moment's notice, which will also brings the end of time, followed by a timeless recreation of heaven and earth (i.e. the universe) ².

Footnotes

¹ I meant here from the point of view of the Biblical author, who didn't have the notion of the Trinity. I also meant PUBLIC revelation (to many people) so a preview such as the Holy Spirit during Jesus's baptism doesn't count, since at any rate Jesus himself said the Advocate / Helper will come later, after He goes back to heaven. Also, when post NT Christians read OT, they detect Christophanies and in some cases project back the fuller understanding of the Trinity, giving fresh interpretation to verses / passages like:

• Gen 1:26 ("make people in our image")
• Gen 18 (the three men who visited Abraham in Gen 18 as Trinity)
• Christophany in Ex 3 (burning bush) and as the 4th person in the flames in Dan 3:24-27
• etc. (see more from the Blue Letter Bible article Do we find the doctrine of the Trinity in the Old Testament?)

² Not all Christians agree on the nature of the new heaven and earth. But given that God's being is outside time & outside the universe, and that He upholds the universe's very existence, it's feasible for Him to do so. Christianity teaches that the universe and everything in it (including human beings) is totally gratuitous in the sense that it does NOT complete God in any way.

³ To commenters who said that this way of understanding the Holy Spirit is impersonal, I would invite them to take a step back and see the history of theology of the Doctrine of the Trinity starting with St. Augustine. The understanding of Holy Spirit as Love between the Father and the Son had a long, established history. I believe Protestants have been neglecting this aspect of the Trinity (for unknown reasons that I'm quite interested to research), as well as impoverishing the understanding of the Trinitarian indwelling in every believer's heart (see Lecture 12 in the supporting material section below).

Supporting material

1. St. Augustine's On the Trinity XV.18.32, 19.37, quoted in EWTN article St. Augustine: Holy Spirit, Gift of God's Love:

   Wherefore, if Holy Scripture proclaims that God is love, and that love is of God, and works this in us that we abide in God and He in us, and that hereby we know this, because He has given us of His Spirit, then the Spirit Himself is God, who is love. Next, if there be among the gifts of God none greater than love, and there is no greater gift of God than the Holy Spirit, what follows more naturally than that He is Himself love, who is called both God and of God? And if the love by which the Father loves the Son, and the Son loves the Father, ineffably demonstrates the communion of both, what is more suitable than that He should be specially called love, who is the Spirit common to both? For this is the sounder thing both to believe and to understand, that the Holy Spirit is not alone love in that Trinity, yet is not specially called love to no purpose.

2. Summa Theologiae First Part, Question 37: The name of the Holy Ghost: Love about how the mutual Love between the Father and the Son should be conceived as a Person, not simply love as a "unitive force". From Article 1, Reply to Objection 3:

   The Holy Ghost is said to be the bond of the Father and Son, inasmuch as He is Love; because, since the Father loves Himself and the Son with one Love, and conversely, there is expressed in the Holy Ghost, as Love, the relation of the Father to the Son, and conversely, as that of the lover to the beloved. But from the fact that the Father and the Son mutually love one another, it necessarily follows that this mutual Love, the Holy Ghost, proceeds from both. As regards origin, therefore, the Holy Ghost is not the medium, but the third person in the Trinity; whereas as regards the aforesaid relation He is the bond between the two persons, as proceeding from both.

3. International Catholic University course The One and Triune God study materials:

   • Lesson 6: Two Processions and Three Persons
   • Lesson 7: Intellectual Generation of the Son
   • Lesson 8: Spiration of the Holy Spirit
   • Lesson 9: Internal Divine Relations
   • Lesson 12: Missions and Divine Indwelling

4. CCC 684 about the progressive revelation:

   Through his grace, the Holy Spirit is the first to awaken faith in us and to communicate to us the new life, which is to "know the Father and the one whom he has sent, Jesus Christ." (John 17:3) But the Spirit is the last of the persons of the Holy Trinity to be revealed. St. Gregory of Nazianzus, the Theologian, explains this progression in terms of the pedagogy of divine "condescension":

   The Old Testament proclaimed the Father clearly, but the Son more obscurely. The New Testament revealed the Son and gave us a glimpse of the divinity of the Spirit. Now the Spirit dwells among us and grants us a clearer vision of himself. It was not prudent, when the divinity of the Father had not yet been confessed, to proclaim the Son openly and, when the divinity of the Son was not yet admitted, to add the Holy Spirit as an extra burden, to speak somewhat daringly. . . . By advancing and progressing "from glory to glory," the light of the Trinity will shine in ever more brilliant rays. (St. Gregory of Nazianzus, Oratio theol.,5,26 (= Oratio 31,26):PG 36,161-163.)

In the beginning was the Word, and the Word was with God, and the Word was God. - John 1:1

And the Word was made flesh, and dwelt among us - John 1:14

I came forth from the Father, and am come into the world: again, I leave the world, and go to the Father. - John 16:28 

There are a great many additional bible verses which declare the pre-existence of the Son of God who took on flesh and became the Son of man at his birth. God is triune in nature and never changes or becomes anything.