There isn't an official supported version of the UNC/FAT add-in for W2K3 Server 64 bit. It hasn't been tested by Microsoft.
I did, however, talk to a developer of the add-in who states it should work on a 64 bit system. What that means is this is untested and if you choose to install the add-in you are doing so at your own risk.
I'll be submitting a DCR to see about updating (or at least testing) the add-in for x64 systems.
Paul Nystrom - MSFT