It is very unlikely you will be able to do this. The actual hardware isn't the problem. You would need to get a module and all the related parts from a much newer car, since all the OEM equipment prior to 2005 (for the Bonneville) was analog. As of December 31, 2007, OnStar will no longer support any vehicle unless it is equipped with dual mode hardware. The only vehicles with analog/digital ready wiring (in the Bonneville line-up) are the 2003-2004 and these need to be dealer upgraded before the end of the year to continue working. The 2005 came equipped with dual mode hardware.
Your bigger problem is that even if everything is installed and functional from a "technical" standpoint, I suspect it is very unlikely that OnStar will allow you to activate the equipment on their system. Without this, you basically have a car which weighs about another 7-8 pounds. I would certainly clarify this with OnStar before going any further. If it'* like everything else GM has told me in the past then you can expect to hear the line I keep getting - "I'm sorry sir, but if it didn't come with that from the factory, you can't add it".
As a further note, there are significant hardware differences in the 00-02 as compared with the 03-05. The 00-02 Bonneville uses two modules whereas the 03-05 is a single unit. Because of this, there are definite wiring harness differences which is why I suspect there is no upgrade available for the Bonneville prior to 2003.
StingRay Bike w/cards in the spokes, Weeeeeee!