Jim Plunkett won two SBs with the Raiders after developing with the Pats and spending time with the Niners (Plunket was basically a bust in New England with his 164 career TD to 199 INT 52.5% career completion percentage. He played well enough and the raider defense was very good. John Madden, the best winning percentage of all time didn't hurt).
Joe Thiesmann was acquired by Washington via trade, although he was playing in the CFL (came from CFL)
Doug Williams developed with TB and won with Washington (was good in TB and was good in Washington, but never great)
Steve Young was acquired by SF via trade (was a bust after 2 years in TB 11 TDs / 21 INTs / 55% completion. Bill Walsh saw something in him and the rest is history)
Brett Favre was acquired via trade, although he never really played for Atlanta (thank you Jerry Glanville!) (Glanville didn't want him drafted and made sure he had no chance to play (0/4 2 int and one sac) - Altanta got a 1st rounder for him)
Trent Dilfer was acquired via trade (considered one of the worst QBs to win a Super bowl - the only QB to win a Super Bowl and be let go after the Super Bowl win - The Ravens knew something)
Brad Johnson was acquired via FA (I believe - yes, free agent) - people talk bad about Brad Johnson but he had some good seasons with the Vikes, the Redskins and the Bucs (pro bowl, several player of the month, career high 4000+ plus yards and 90+ passer rating)
Drew Brees was acquired via FA (he became a free agent because of injuries, NO took a chance, and the rest is history)
Peyton Manning was acquired via FA (he became a free agent due to a neck injury, Indy drafted Luck (a once in a decade type of QB), and he chose his destination). He was basically not a QB when they won the Super Bowl)
Basically, its not that uncommon for a team to win a SB with a QB they didn't draft. The trick is that you have to acquire the right one.