Rainer Willems from our German office has come up with a way to allow you get formulas into your Excel outputs from an RTF template, its a neat idea. It needs some thought up front about where data is going to be rendered in the Excel output, but thats easy enough to find out, just run the template to Excel et voila.
You can check out the details here
and for German readers
Just one drawback, that Rainer mentions, once down this path with an RTF template its forever a 'Excel' output template. Thats unless you are on 10.1.3.4 Standalone or above when your template becomes self aware of what its being run for e.g. PDF, RTF or Excel. Then you can add some logic e.g if output != 'Excel' then hide this section of the template end if. More on that another day.