To add an extra credit question to a quiz:
1. Add the question, and make it worth 0 points.
2. Put setting to manually grade it.
3. Manually add the desired points.
4. Those students who do not earn 100% will get the extra points you manually issue.
5. Those students who already have 100% will get the 100+X score
There is no easy way to grade those extra credit questions, if not written as open-ended questions. You would then have to manually open each assignment to scroll down to that question/answer and manually issue points, save, and then do the same with the next submission, etc.
In sum, there is no easy way to do this, but any question can be tweaked to
work as extra credit, if you can score it manually.
Adding Extra Credit Assignment
In order to add extra credit, create an extra credit category (with the extra credit checkbox selected) and place the extra credit assignments (which do not have the extra credit checkbox selected) inside of it.