I have not seen an easy option to import all the questions from a quiz to an item bank at once. With Classic Quizzes, a Question Bank would be created that was essentially "Untitled" that contained all questions created within a quiz that were not set to be in question banks. That still required sorting the questions into their respective banks.
I would suggest, starting at this point, that you create the item bank first. Then go to Item Banks in your course menu (if not there, you have to add it from Settings) and open the desired bank. You can create questions in the item banks. This will automatically attach them to the bank, which can then be pulled into any quiz. Make sure that you select "Share with XYZ Course" when creating the bank. It makes for easier sorting to find your banks. For your existing quizzes, you will have to add them to banks one by one. Create all the banks you think you will need first, then go into your existing quizzes to add the questions. It will unfortunately be a bit more time consuming at first.
I once had a teacher (back before New Quizzes were released) that set his own goal to create 2-3 questions every week. Not 2-3 per bank, but just 2-3 total. Sometimes, it might be a single math problem, but he would add three variations with different numbers. By the end of the year, he had multiple developed item banks. Across several years, he had banks with lots of questions. Your goal of objective-based banks is great and will increase both the validity and reliability of your assessment results. The ability to pull random questions from a bank means that students sitting beside each other will likely have two different sets of questions. The more questions in a bank, the more variability quiz to quiz. And this works best when banks have a narrow focus as you are aiming to do. I love sharing that idea, because it is such a simple but effective way to build out the banks of questions.
