[f5fstop]

Interesting question, not sure, but I would say no.
In all their activation instructions, they tell you to activate via the button on the Onstar system. This is the way they identify your vehicle, and place the account in your name. It is also the time that you set up your PIN number, for use when outside the vehicle (locked keys in vehicle, locate vehicle in a parking lot, located stolen vehicle, etc.)
Without the PIN, and without the system already setup, even with the magic police code, Onstar may not legally be able to activate for an unknown person.
So, if your vehicle is stolen, you would "probably" need the thieves to activate the Onstar button; which I believe would be rough.

Best advice, call 'em, but I do believe the Onstar button has to be pushed in order to recognize the vehicle for the initital setup of the system for each specific vehicle.