The credit card key is for keyless entry and ignition. It is more than just a key, it is a different ignition module and both front doors and trunk have atennas to detect the key and buttons to unlock the car without removing the key from your pocket.
So in short, if you don't have keyless entry/ignition you can't have it...unless you want to pay an astronomical amount of money and it would be far cheaper to just trade in your Sport model for a GT.
