Theorically, yes. Realistically, no. The heatsink on Apple's OEM RAM is larger than the standard FB-DIMM. If you use a FB-DIMM not certified for the Mac Pro you a likely to get memory errors and generate noise from the RAM as well as kick on the fans more often.
Best thing to do is play is safe and buy RAM that has been tested for use in the Mac Pro and has the beefier heat sink. There are many 3rd party sellers that have such RAM or you can buy direct from Apple.