The quick answer is no you can't.
The longer answer is a shaky maybe, but for starters it involves getting a ROM dump from a Mac 8800 and flashing your PC card. The first problem is finding the Mac ROM image. Then you have to flash the card in Windows, and as the poster above mentioned, your particular 8800 has to have enough room for the EFI64 firmware. Doing so, you're always taking the risk of bricking your 8800 should something go wrong.
Beyond this, the power cable on your PC 8800 may not match the Mac Pros motherboard connectors. You may have to splice some cables.
If this sounds like a royal pain in the a** it is. People have migrated their PC 8800s over to their Mac Pro and have posted their results here. You can do a forum search for the results. IMHO, it's better to just spend the $200 for a bona fide Mac 8800.