You can hook up both the XM and Sirius add-on units and subscribe to both networks, with XM you would need the additional NavTraffic subscription.
The Sirius website has a list of Sirius NavTraffic compatible equipment, that list is very short right now, one unit only and that's for Alpine head units.
Taken from their website:
Q. Which navigation systems are capable of using SIRIUS Traffic data?
A. The first available SIRIUS Audio plus Traffic Tuner is the SIRIUS SIR-ALP10T. Compatible with Alpine SAT-READY headunits and the Alpine NVE-N872A Traffic Ready Navigation system, the SIR-ALP10T is distributed through Directed Electronics. The Alpine products are available through Authorized Alpine Dealers nationwide.