You can actually do a lot to customize the rendering extensions in SSRS...

here is a link to the MSDN documentation for 2008 (there is a 2005 page as well) for modifying the CSV extension.

http://msdn.microsoft.com/en-us/library/ms155365.aspx

Also at the bottom of the page there is a link to the section on modifying the rendering extensions themselves... it's actually fairly easy, all you have to do is make the appropriate changes to the RSReportServer.config xml document on your report server. Just make sure you make a backup first!

-Luke.