GMC DVD Navigation Question...

I just purchased a new 2004 GMC Envoy SLT with the DVD Navigation and was wondering if anyone has this system and if so can seperate aftermarket maps and stuff be used with this?
I pick-up the vehicle tomorrow but I already understand that the mapping software for Canada is pretty slim in the cities and real bad in the north so I would like to find something aftermarket so I can update the more remote areas of Northern Ontario if possible. Can anyone recomend a company that has the GPS software compatiable with GMC?

Appreciate any info.

Congrats on the new auto. I dont know any companies off the top of my head but I can ask my buddy. He is really into those kinds of things. But have you tried asking your local dealership if they have any upgraded software??

Hey thanks for the quick reply Ammar.
I did ask and they told me there is not much from GM on Canada at this time due to our Gov. not spending the cash to update the land mapping here since the 70's but is upgradeable via the dvd when future updates from GM are out.
I have a couple of hand held Lawrance systems that are upgradeable via aftermarket so I was hoping that this system would be as well.