Here's my understanding.
All smartkeys are unique - no two are the same.
The keys are not programmed to work with the car. Instead, the car is programmed to work with the keys. I recall a maximum of 10 keys can be programmed into the car.
So unlike a conventional car, in which all keys look exactly alike to the car, every smartkey is different. The car can then decide to accept or reject any given smartkey.
In theory, you should be able to program the new key into the car. In practice, I don't know if there are limitations, dealer training issues, etc.
There are some neat options available as a result. If you lose one key, just have it changed to unacceptable in the car's computer - no need to replace all the remaining keys. Perhaps you could even have one key programmed to start two (or more ) cars...