Hello Eric,
There is a VI to do that, it is inside the RFSG LLB located usually at:
C:\Program Files\National Instruments\LabVIEW 8.2\instr.lib\niRFSG\niRFSG.llb\niRFSG Get Fgen Session.vi
Is there a special reason why you would like to use this handle that you were not able to do with RFSG driver?
Best,
Gerardo O.
RF SW Engineering R&D
National Instruments