Anything's possible, but it would be what's called "prohibitively expensive". I'm sure that you couldn't even buy all the pieces new from Ford anymore, you would have to get all the lines, wiring, and controls from a 93-up car. Yes, the last of "older style" Birds and Cougars has the newer generation system. It would be ALOT of work to try to switch everything over, this would be a case where it would make more sense economically to sell your 89, and buy a 93-95 car if that's the ABS system you want. I would venture to say that it is a more reliable and simpler system compared to the earlier style.
cheers
Ed Nicholson
SCCoO