I did it on my old car. Pretty easy job (depending on the car).
you can find some remote central locking kits on ebay. Mine was about 25€ delivered.
If your cars locking pins are on the top of the door then it should be easy enough. If they are by the door handle then it may not be possible.
It just a case of removing the door cards, fitting a solenoid to the inside of the door, feeding wires to the other side of the car under the dash and finding a Wire with constant + and an earth. If you have 4 doors then its a lot more work removing panels, carpet and maybe seats to feed the wires.