I have 2003 Chevy Monte Carlo SS with the factory installed XM Radio with the factory XM Antenna mounted on roof,
I purchased a Kenwood touch screen which I want to use with my Sirius account,
my question is will the factory installed XM antenna work for Sirius ?

Yep....You may have to get an adaptor harness to get it to plug into the Kenwood. I have had luck with splicing ends together. If you're going to go this route, make sure you solder the center conductor, tape it, and use a long piece of wire, strip it back and wrap it around what you have cut to make sure it is sheilded properly. Solder the new wire to the existing shield as well