So you've decided to incorporate extra credit into your gradebook?
You may select any item in your gradebook to distinguish as 'Extra Credit'.
Select an item, then 'Edit Settings'.
Open the item's settings, scroll to 'Parent Category' part of the settings, then check the box for 'Extra Credit'.
Once you return to the gradebook, the grade item should have a little "+" sign next to it. Feel free to connect with the following link for further details on how extra credit works.