The Aux in adapter Pioneer uses also has an IP-in connector. So you can daisy-chain your XM to the aux-in adapter. (which in it's turn is connected to the IP connector on the HU). Everything then runs via the IP bus.

Once this whole setup is connected, I suspect a new setting on your HU will show up, which will let you choose which channel you'd like to hear the sound of.

Just for the record : I don't have an XP module, but I HAVE got an IP aux-in adapter, so I've seen this extra IP-input on the aux-in module.