Q. When did Zipporah convert to Judaism? I have not been able to find a source that would provide that information.
Zipporah was the wife of Moses. We learn early in the book of Exodus that when Moses fled from Egypt to the land of Midian, he found refuge in the home of a man named Jethro, who was a “priest of Midian.” Moses worked for him, tending his flocks in exchange for his own keep. Moses eventually married one of his daughters, Zipporah.
The Bible does not say specifically that Zipporah embraced faith in the God of Israel when she married Moses, but we do have one slight indication that she probably did so. In a passage that is admittedly strange and difficult to understand, in order to keep God from being angry with Moses, Zipporah circumcised their son Gershom. So somehow she knew that God expected this of his covenant people, and she was prepared to do it.
That is all we really have to go on. There are some things that the Bible does not tell us as much as we would like to know about. But I think we do have enough to go on to conclude that Zipporah did come to share Moses’ faith in the God of Israel.