If memory serves, you could drag the columns separately into the detail area and hide/supress them. Then sum them on a "summary" line (or "Total" or some such in the UI). Hide those as well. Then create a formula field on the summary line that sums the summed fields on the summary line.

That's as best as I can recall and its been more than 10 years since I played with CR.

Hope that helps. If this sounds reasonable and you need more detail, lemme know.