Yeh of course u can, just like joogie said with enough time and money. The pod on the key can clip straight on the original key. Get central locking out of a calais or somthing or statesman and put them in, tricky and annoyin job. but if you are keen. It can work easily. eithr find same key pod to central locking or pay to get it reprogrammed
I would think it would be fairly difficult to do, as the VR onwards Heads communicate with the BCM on the car. But as others have said nothings impossible.
seeing as you have central lockin already u will have to go with the vn statesman central locking. holden may do it for you but very costly, i know it can be done but not sure who around your area can do it.