Moodle allows for Extra Credit graded items.
Please follow this guide to create gradebook categories.
The new category will appear at the bottom of the Gradebook setup. Scroll down to find it. Once you've found your category named "Extra credit" or whatever you named it, click "Edit", then "Edit settings"
Check the checkbox next to "Extra credit", then click "Save changes"
In your Gradebook setup, you'll be able to tell if a category is extra credit, if the category's point's total has a "+" icon next to it.
Add your items to the section, and anything a student completes will be additive to their grade.