Warren Moon, Joe Theisman, Doug Flutie, Adam "Pacman" Jones just to name a few.
Okay even though you threw in a few pre 90 players (Moon, Theisman), I will start from there. Theisman , based on my research , failed in contract negotiations with the dolphins and went to the CFL instead. It looks like the Redskins got the rights to him and he came to the NFL. Looks to me he used his stint in the CFL as a negotiating ploy.
Warren Moon, interesting, I forgot in his time , a black quarterback was not too acceptable and he refused to consider switching positions. Despite a successful collegiate career, he was not drafted by the NFL, most likely due to his insistence on being a QB. Went to the CFL to play his chosen and deserved position, an injustice indeed.
Jones ?? looks like he used the CFL as a pit stop because he was a persona non grata in the NFL at the time.
Flutie , class act(and in case I didn't mention, I thought Moon was also). However, he could have easily finished his career in the CFL. He was lucky to get back to the NFL, although once he got back there, he exceeded expectations.