I know how to make quizzes from question banks, but is there any way to move questions from existing quizzes into a question bank?
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How to add questions from a quiz to a question bank?
The method used to get question banks created would be to create a quiz export where the quizzes reside, then import that newly created export .zip file back into the course. This will most likely duplicate the quiz, but you can deleted the duplicated quiz and keep the question bank. In addition, since there are quizzes that already exist within the course I would suggest selecting: "Overwrite assessment content with matching IDs".
I would also recommend testing this in either your test or beta environment or in a development course.
For information on How do I export Quiz content? click on the link.
What does the Course Import tool do?
Retrieving data ...