it's probably not possible out-of-the-box, so you should look into building your own rendering extension for this, or maybe you can find a third-party rendering extension that suits your needs.
If you have to build it yourself, I think it would be easier to render to excel 2007-2010 format, as that is essentially a zipped set of xml-files, whereas excel 1997-2003 is a binary format.
Peter Rijs
BI Consultant, The Netherlands