This can certainly be done. I've added it to my 2004 GXP and it works great
. I also had an '02 SE which I upgraded the factory head unit in making it XM Ready, then added the Sirius SIR-GM1 (not the factory XM, but the same operation through the factory head unit). If you have a 12 Disc Player in the trunk, the process becomes a little easier. If not, it'* still possible, just more work. You will have to build the wiring harness for the trunk mount receiver. Additionally, you'll need the XM tuner, antenna, antenna coax and the tuner mounting bracket.
As for the head unit, I purchased one off eBay which was XM capable. Basically, if you have an AM/FM button, you will need to upgrade. If you have a BAND button instead, it will be XM capable.
It is generally quite a bit more expensive to add XM this way, but if you're willing to spend the money there are a few options. By the time I was finished with mine, I had spent the better part of $1,000.00 CDN.
There can be several hurdles throughout the entire process (as I learned the hard way). If it'* really something you want, I can guide you through the process I went through.