Adding Keys (Domestic): Important:

WARNING: This page is about a different car, the 2003 Pontiac Aztek and 2003 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

You may add up to 10 additional master keys or valet keys using this procedure when a learned master key is available.
This procedure adds keys only. The procedure does not erase previously learned keys.
The keys to be learned must duplicate the mechanical cut of the current key.
If more than one valet key is to be learned, immediately precede each valet key with a learned master key.

With a previously learned master key, turn ON the ignition, with the engine OFF.
Turn OFF the ignition and remove the key.
Within 10 seconds insert the key to be learned. Turn ON the ignition with the engine OFF. The vehicle has now learned the new key.