You can add Extra Credit Assignments using regular assignments. It is simple. Following is our vendor's article about it: How do I give extra credit in a course?
Make sure you limit extra credit points to about 10% of total points that students earn from regular assignments.
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Under your course, create an assignments and set the points to "0" in order not to influence the total number of points that students earn.
When grading, you can click on the box under the Extra Credit column, and type desired points even though the points set is "0". This is to not influence the final grade (final points earned by students)
NOTE: Unless you publish the assignment, this column for grading will not show up.