This is a multistep process. Cumbersome, but quick. I just did this for a course.
First go into Classic quizzes and create a quiz using banked questions , if the quiz is already using a bank, create a SEPARATE quiz for each bank ( make sure you name them something identifiable-( ie chapter 1 BANK) (DO NOT BANK THEM AGAIN- SEARCH FOR QUESTIONS and just add them all without accessing a bank). Once this is completed, go into export and export EACH new classic quiz as an individual QTI file (ie. Chapter 1 Bank quiz), download.
Migrate the current quizzes to new format (the questions will be there if not in banks- If that is not working, that is something broke on Canvas end). This step can be done first or last.
Go into the Item Banks of the course - on the menu options. Create a bank (name does not matter as it will be overwritten by the name of the QTI file).Click on the name of the Bank (we use 1 for ease of finding). In the top most right corner of the screen, click on the there dots and chose import content. Find or Drop and drag the downloaded QTI file. Wait for it to complete the import- this may take a moment. share with the course. You will then see the new bank with the bank name as it was created and the bank named 1 is gone. (Share this bank with whomever else you want to have access to it, and make sure to change them to can edit if they will have that ability to change, add, and delete questions).
From here, go into quizzes, create your quiz under "new quizzes" and complete the steps to create the quiz, importing banks as needed. Again, this only is for Banks in quizzes, migrating a classic quiz to new quizzes that is straight questions requires nothing other than migrating and being patient.
Hopefully this. helps!