Currently, Canvas does not have a default way to add extra credit. Below are two easy ways to add this yourself.
You can add a zero-point to a weighted assignment group to add extra credit. To do so, follow the steps below:
Another method is to add points to a student's grade for an assignment, such as giving them a 25/20. If you add extra credit this way, it is recommended to add a comment in the feedback for the assignment that you added additional points for extra credit.
For a walkthrough of these options, check out the video below.
For more information, please see Extra Credit Guide by Canvas.