I know you can program a new key for central locking unlock using a working key in the ignition on most cars, but you can not reprogram a key to disable the immobiliser and start the engine using this method
There are two completely separate systems incorporated into a modern car key. One is much more secure (immobiliser), works independently and has to be programmed separately from the other (central locking).
i've been searching through because i'm after another key, bought the car with only one key. got a price from my local fiat dealer of £95 for a non remote and £175 for a remote!
i'm going to look after my one key very well i think!