For everyone ending up here after googling if you can use an xbee module with the Arduino due & wireless SD shield:You can, I've tested it just now and it works. I didn't test the SD card yet but xbee communication is definitely working.
But as it is an officialy supported shield, there should be no problem, right?
As far as i figured out, the Wireless SD Shield works has 3.3V Signal levels, is that right? In this case there should also be no problem with the signal voltage.