This part is done directly in your Excel template. The best way to understand how this works is to look at the Opportunity Demo.xlsx file supplied with the add-on.
If you run the report, you'll see that there are totals at the bottom of the report and that the last column on the right, titled Commission is calculated.
How to insert totals/calculations at the bottom of the exported data range
If you open the template (File > Edit Template...), you'll see that the template is set up to have the data be inserted starting at row 4 1.
To be able to create formulas that will stick, you need to add them NOT to the next row (row 5) but to the next one (row 6 - 2) or lower and whatever you calculation is, you need to set it up as if the data was on rows 4 and 5. For instance, if you look at the formula in cell J6, it is =SUM(J4:J5) 3.
That's the only tricky part: you need to skip one row (row 5, here) and reference the 2 rows in your calc (Rows 4 and 5 here).
How to insert totals/calculations on the sides of the exported data range
This is simpler. If you look at the formula in the Commission column 4, it simply takes the cell to its left and multiplies it by 0.25. You can do this either on the right of the exported data range or on the left. Simply reference the cells on the same line that need to be part of your calculation.