@DianaNguyen -
If you are using new quizzes, can you set the points for the quiz to 0 and then have the questions worth points so that the quiz counts points, but the weight in the gradebook for calculating the grade is 0 points for the quiz (i.e. students would receive 5/0) this would work for weighted or unweighted gradebooks. I do not know about this method because I do not use new quizzes.
The way I create extra credit quizzes is more work on my part, but I know it works. This method only applies if you are using weighted gradebooks
I use classic quizzes, so it may be easier in New quizzes using the first method described. For this method with weighted gradebooks, I create an extra credit assignment group and weight it at 0%. I then create all extra credit quizzes in that group with regular points. After the grading is complete on the quiz, I create a regular assignment worth 0 points in the assignment group associated with quizzes (or where ever it will be in your course). I then export the gradebook and copy the actual quiz scores over to the assignment that is being used to report those scores.
it is more work than I want, but I do not use it too often.