Hi,
No the RPM of the MGX is a fully capable router that does everything a router can do, all that you should do is to logically cross-connect (using addcon) the PVCs to its specific switch subinterface (which is an ATM interface which is connected via the backplane of the MGX to all the other cards in the MGX), if for example your PVC is coming from the FRSM or AUSM you'll have to do cross-connection from the RPM to the FRSM or AUSM.
After you have configured your PVC, you can freely configure whatever routing you require with no constrains.
HTH, please do rate all helpful replies,
Mohammed Mahmoud.