In case anyone else needs this answer and comes upon this page, we had success with these instructions, which I'll copy/paste below.
Import into New Quizzes:
- Go to the Quiz index page and click +Quiz. Select the new quiz engine option. Fill out the assignment settings and click Save. The new quiz screen will display. Please note with this option you can only import into new blank quiz.
- On the quiz edit screen click the options menu (3 dots) and select Import Content.
- Note: If the quiz has existing questions, the Import Content option will be grayed out.
- In the dialog box that appears click Browse and locate an export package for the quiz.
- Click Import. This will begin the import process. Continue this process for each quiz you exported.
- (Optional) If you wish to add questions to an item bank, edit each question and select Item banking to add the question to new or existing item bank. Or you can import directly into an Item Bank.
Here's the official Canvas guide: https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-import-a-quiz-from-a-QTI-package-in-New...
Note that I just helped an instructor export his questions from a New Quiz Build page (three dots > Export ), download the folder from the Exports tab, unzip it, open and edit it in the Notepad app, save and re-zip the folder, then import it with the instructions above, and it worked!