Some of our instructors have the need to provide extra credit via a quiz. Students would earn specific points based on their correct answer and receive extra points based on their score but it would not increase the course or assignment group total points. I envision this as a checkbox option in Quiz creation.  Mark this Quiz as extra credit. Then it would behave in the manner described above.
Here is the old thread on this. I could not find a new version here but maybe I am missing it.
An alternative to this would be to allow questions themselves to be marked as extra credit and that would allow the flexability to have specific questions within a Quiz or the whole quizzes set of questions marked as extra credit.