If it's not a master, then if you ever lose that key then you're screwed, sorry (new ECU). Hopefully it is a master though. If it's a master, you can programme in new keys (you'll need to buy the fob/transponder etc, e.g. through eBay, see sticky thread for details).
Basically you can only programme new keys with a master key. If you don't have a master key, you're stuck with the keys that you've got, as I understand it.