I simply use the Excel template to add my own total rows where I desire them and in the format I desire them in NPrinting.
Your SUM() would include the first cell where the label is AND the row below it which means (all future rows that will be populated when the report is run. Note I bring the fields over 1 by 1, rather than just dropping in the table as a single object. Which adds the benefits of being able to display columns in different orders, or with different formatting if desired.