I appreciate the previous solution to this problem. Let me offer another option to converting question banks to item banks.  I have also found a way that seems to work and it's pretty quick and easy. Check out my video Converting Classic Question Banks to New Quiz Item Banks video

STEPS:

1. Create Classic Canvas quiz
2. Create and name a "question group" but DON'T link it to a bank
3. Find questions from the question banks and add them to the question group
4. Save quiz
5. Migrate to a new quiz.

DONE! Now you have a new quiz and item bank based on the classic question bank.