Hi all I'm wondering if anyone can tell me what I can do...

I have a 2002 ford KA with central locking but bought it without the remote central locking key all I have is the spare key which is the master key with the red dot, it doesn't work very well either meaning it locks the car fine in both passengers and drivers doors but when unlocking the car the drivers door doesn't work only the passenger door let's me unlock, so I was wondering if I buy a replacement central locking key fob suitable for my car will i be able to program this fob just to unlock the car? I know I won't be able to use the actual key to start the engine or in the doors but I was hoping I can get the fob programmed to remotely unlock the car and just carry on using the original master spare key for the ignition so basically have 2 keys one as just a fob and one to start the car?? Thanks!