Sometimes, when adding a quiz to a new card set, you may need to reuse a question from another set. To do so, add a new quiz (1) and then, edit a question (2):
In the question's title field (3), begin with an "@" and then start typing your search terms to find the question you want. From the search results, click on the question you want to add it to your quiz (4).
You can then edit the question as if it was new, without affecting the original one.