So...as a solution to combine the two...you can just merge the Google Contact, with your regular contact. But the catch is that whenever you disconnect or reconnect (actually, I am not really positively sure when this actually happens, but I know that it does) somehow these two 'contacts' get split again, and you again have two contacts for the same person, your original, and the Google Voice one.
An interesting thing to note, is that when this occurs, there is a left-over in the original N900 contact. What is left over, is a contact field just called "Google Voice." When you perform a 2nd, or subsequent merge, this field disappears, and is replaced with the standard "Google Voice IM" and "Google Voice call." Now, of course, the default fields are ALWAYS present, I believe as long as you have the plugin installed(not sure if disabling the account removes it,) these being, "Call through Google" and "SMS through Google."
at one point I was using Google Voice with a SIP account, using the SIP # as the callback number, and I was receiving calls and txt's just fine with this method....although I am no longer doing this because call quality was not consistent (my idea was do snag a mobile broadband SIM, and put it in the N900 for a $50 a month data plan, with GV as phone and txt service) as well as the fact that while moving, say in a car(motorcycle actually,) calls were impossible due to my location being constantly in flux.