Total Column:

1. Create a column manually and call it "Extra Credit." 
2. Make the Points Possible "0" and click Submit.

Any points you add to this column will be extra credit, and the total points possible will remain the same.

For example:

If the student earns 10 extra credit points, you will type 10 in the Extra Credit column as their grade.

This will add to their grade, but keep the total points possible as the same amount.

In the image below, the Total Points Possible is 150 points, but the student earned 160 (including 10 points Extra Credit)

Weighted Total Column:

1. Create a column manually and call it "Extra Credit."
2. Make the Points Possible "0" and click Submit.
3. Click Create Calculated Column and select Total. Give the column a name that will help you and the student identify that this is the final course grade (i.e. Final Grade). Scroll towards the bottom of the page until you see Select Columns. This will allow you to add the Weighted Total grade with the Extra Credit column grade together. 
4. Under the box Columns to Select, click and move the Weighted Total column that is calculating your students weighted total grade and the Total column you created called Extra Credit. Both columns should be moved to the Selected Columns box to the right (you can use the small gray arrow to move the columns). Click Submit.