You can work with XDCAM EX footage if it's in a QuickTime wrapper and only on a Mac with the proper XDCAM EX codecs installed.
So, if you have FCP then you'll have the XDCAM EX QT codec and Resolve will be able to use it.
You'll have to use either XDCAM Browser or FCP7 to re-wrap native MP4 XDCAM EX files to QuickTime in order for them to be compatible with Resolve.
If you're on Windows, you're out of luck.
This is one area where I wish Blackmagic would expand it's supported codec support. It would save me a lot of trouble if they supported XDCAM EX natively in it's .MP4 wrapper on both Mac and PC.
-J