Yes, even have the owner's manual
In answer to a previous question you had, per the electrical schematics, it does appear it could be a plug and play to switch out the current radio with a Nav unit (expensive plug and play I will add...remember, this is a $1900.00 option). It appears that the inputs to the Nav are on the same pins as the inputs for the optional radios (such as vehicle speed input, power, ground, etc.). Needless to say there is an extra input for a GPS coax on the Nav.
Only thing that might have to be added is the external GPS antenna. Per the service manual, it will be located under the top cover of the I/P near the window.
However, I do know that some of the newer shark fin antennas (and the Hummer has one) can be used for XM signals, GPS for Onstar and GPS for Nav units. The one on the H3 is currently used for XM and GPS.