This question is difficult to answer because eternal progression is hard to define. Consider this quote from the Encyclopedia of Mormonism:
The principle of eternal progression cannot be precisely defined or comprehended, yet it is fundamental to the LDS worldview. The phrase "eternal progression" first occurs in the discourses of Brigham Young. It embodies many concepts taught by Joseph Smith, especially in his king follett discourse.
If you mean specifically the potential for man to become like God, I can give a fairly specific answer.
A detailed revelation received April 1st, 1832 and recorded in D&C 76 increased Joseph Smith's understanding of the afterlife and man's potential to progress.
LDS Church History states that Joseph Smith and several other men were pondering the significance of John 5:29, which speaks of the resurrection, when he and Sidney Rigdon were shown a vision.
In the original record of the vision, Joseph Smith prefaced the revelation saying how it increased his understanding:
through the power of the spirit our eyes were opened and our understandings were enlarged
This revelation, among other things, speaks of three degrees of glory that the dead will inherit in the afterlife. Of those who fully follow Christ's gospel, the record says:
58 Wherefore, as it is written, they are gods, even the sons of God—
59 Wherefore, all things are theirs, whether life or death, or things present, or things to come, all are theirs and they are Christ’s, and Christ is God’s.
Interestingly, this references earlier scripture. Possibly Ps. 82:6; John 10:34; or 1 Cor. 8:6.
In any case, this revelation increased Joseph Smiths understanding of eternal progression more than revelations and scripture prior to April 1832.
More specifically, Joseph Smith stated in his preface:
From sundry revelations which had been received, it was apparent that many important points touching the salvation of man had been taken from the Bible, or lost before it was compiled.
Regarding Kolob, the earliest reference to it in the Joseph Smith Papers project (which intends to publish all documents written by Joseph Smith) is from circa July to December 1835, the time when Joseph Smith began translation of the Book of Abraham.
Therefore, I think the idea of a place called Kolob was probably not known prior to the translation of the Book of Abraham.