Joseph Smith did not add any more words to JST, Genesis 50, that were quoted by Lehi.
The ninth characteristic of the choice seer could be considered an extension of both the fourth and the eighth one, since they both deal with the writing of the fruit of the loins of Joseph of Egypt. Because it deals more with the message of those writings, it will be considered as a separate characteristic.
“The words which are expedient in [the Lord’s] wisdom [that] should go forth” (v. 19) were “the words of them which have slumbered [the Nephites]” (2 Nephi 27:6) who were destroyed. “The things which are sealed [the sealed part of the plates]” (2 Nephi 27:7–8) were not translated, but “shall be made known unto future generations” (D&C 5:9). The first words “shall cry from the dust; yea, even repentance unto their brethren, even after many generations” (v. 20). Their coming forth will fulfill Isaiah’s words: “thou shalt be brought down, and shalt speak out of the ground” (Isaiah 29:4; see also 2 Nephi 27:16). Thus Joseph of Egypt and Isaiah are two witnesses of the coming forth of the Book of Mormon, and again designate Joseph as the choice seer of the latter days. The Lord promises to make the words of the Book of Mormon “strong in their faith, unto the remembering of their covenant” (v. 21). As Moroni promised: “he will manifest the truth of it unto you, by the power of the Holy Ghost” (Moroni 10:4).
There is no other person in these latter days to whom all nine of these characteristics could apply.