Some of the Jataka Tales are supposed to be about the Buddha in prior lives so the entire collection is touted as Buddhist by many. However, other sources will say they are tales handed down through oral tradition that begins before recorded history. My guess is many of the tales were already favorite tales that Buddha added two cents too, and a few were new, but that is only a guess. Is there any good evidence about the origins of some or all of these tales?
The Jataka Tales are moral tales, probably written down in this form in the 4th century, ie probably a 50 to 150 years after the life of Gautama Buddha.
Yes, many of these stories predate Buddha. Some of the stories are variants of stories believed to have been told by Aesop in the 7th century BC, for example.
I don't think Gautama Buddha added anything to these stories. The claim that they are about his earlier lives is probably something that arrives only well after his death.