The keyless entry function would have to be programmed into the computer module when the car was produced. If the car wasn't equipped with keyless as an option when new, then you would have to purchase a new PCM and have it programmed to accept keyless entry. The PCM is kind of a one shot deal, it can't be reprogrammed to accept keyless entry.
The least expensive avenue for you to take would be to purchase an aftermarket keyless entry out of the JC Whitney catalog for $39.95 and install it yourself. This unit is very basic and will not honk the horn or flash the lights, but it is very reliable and will lock and unlock the car as well as 'pop' the trunk."