It was pretty rare, but there were a few. If you'd like to read more, check out Stella: One Woman's True Tale of Evil, Betrayal, and Survival in Hitler's Germany by Peter Wyden. Stella was a Jew and initially used fake papers to become a U-Boat--a hidden Jew living in Berlin using, usually, false papers to pass for a non-Jew. She and her parents were caught and imprisoned. The Gestapo made her an offer: they'd kill her parents unless she agreed to work for them finding other U-Boats in Berlin. She agreed, and became a "catcher", the term for people who did this sort of thing.