As mentioned above, GeoRSS and KML are not supported. However, someone has written a GeoRSS interpreter for Bing Maps v7. And since KML files and GeoRSS files are both XML, so you can easily convert between the two using XSL. Some hunting shows that many people have talked about it, but I can't find any actual XSL file that does it. I did, however, find one the did the opposite, converting GeoRSS to KML, which should give you a good idea of where to start if you wrote your own.
If you only need to do this once, you could use to following two website to convert KML to GeoRSS via CSV:
- A KML to CSV converter using XSL.
- A CSV to GeoRSS webapp
And a final option is simply to write your own KML interpreter, in the same way someone wrote their own GeoRSS interpreter, per @Derek.