Anything is possible, but aftermarket ABS is not practical.
ABS relies on a sensor on each wheel and a hydraulic pump to 'pump' the brakes, connected to a computer (usually an ABS module that's part of the car's ECU). You would need an ABS module and sensors installed. You can see what's involved here:
http://auto.howstuffworks.com/anti-lock-brake.htm
Better just to pay the extra $250 for the ABS package. As far as ABS goes, a car either has them or it doesn't.