Assuming a belief in God:
The doctrine of eternal progression, which Pres Hinckley said he's not sure we teach, is that man can become a god.
Emphasize. He was obviously trying to find the right words to fit within the milk before meat principle.
Yet the only examples we have of Gods are God and Christ, neither of which can be proven to have ever been a man before they were Gods.
Is there any other documentation of this idea, so that there would be some basis for Joseph to restore this? Or is this something Joseph dreamed up all by himself?
Biblical Theosis (among other things).....
John 10: 34-36 (Ps. 82: 1-8) We are divine gods already
Acts 17: 28-29 We are the same type of being as God.
Romans 8: 17 We will inherit everything God has.
2 Corinthians 3: 18 We will have the same image and glory as God.
Galatians 4: 7 We are heirs of God through Christ.
Philippians 2:5-6 We are to think that we can be equal with God.
Philippians 3:21 We will have the same kind of body as God.
1 John 3: 2 We will be just like God.
Revelation 3: 21 We will have the same power and authority as God.
"Men are Gods and Gods are men." Clement of Alexandria, The Instructor 3:1
"We have not been made Gods from the beginning, but at first merely men, then at length Gods..." Irenaeus, Against Heresies 4:38:4, in ANF 1:522
"...our Lord Jesus Christ, who did, through His transcendent love, became what we are, that He might bring us to be even what He is Himself." Irenaeus, Against Heresies 5: Preface, in ANF 1:526
All men are deemed worthy of becoming gods, and even of having power to become sons of the Highest. Justin Martyr, Dialogue with Trypho 124, in ANF 1:262
"we assert that not by their communion merely with Him, but by their unity and intermixture, they received the highest powers, and after participating in His divinity, were changed into God." Origen, Against Celsus 3:41
God "made man for that purpose, that from men they may become Gods." Jerome, The Homilies of Saint Jerome, vol. 1 (FC 48), translated by M.L. Ewald, 106
"For as Christ died and was exalted as man, so, as man, is He said to take what, as God, He ever had, that even such a grant of grace might reach to us. For the Word was not impaired in receiving a body, that He should seek to receive a grace, but rather He deified that which He put on, and more than that, gave it graciously to the race of man." Athanasius, Discourses Against the Arians 1:42, in NPNF Series 2, 4:330-331
Orthodox Christians "taught that the destiny of man was to become like God, and even to become deified" Prestige, God in Patristic Thought, 73
"One can think what one wants of this doctrine of progressive deification, but one thing is certain: with this anthropology Joseph Smith is closer to the view of man held by the Ancient Church than the precursors of the Augustinian doctrine of original sin were, who considered the thought of such a substantial connection between God and man as the heresy, par excellence." Benz, E.W., Imago Dei: Man in the Image of God, in Madsen, ed., Reflections on Mormonism, 215-216