Ron,
No, the 2003 OnStar antenna cannot be used as an XM Radio antenna. Understand that XM antennas are actually 2 in 1. They support both satellite and also ground "repeater" reception.
Take a look at this thread:
http://elcova.com/groupee/forums?a=tpc&s=2826088551&f=6706011751&m=307102578
Look at the "magazine" type changer links.