If you are really THAT determined, why not call 800-25-volvo (Volvos literature #) and order wiring manuals for both your 850 and the '96 850, then look up the wiring diagrams for central locking on both and see what the difference is? You (or someone) could then figure a way to wire it in.
IMHO, it'll be more of a pain to do than most people (even most skilled techs) to mess with. I'm sure that with determination, it's possible but most people wouldn't want to do it even if you offer to pay someone else unless they're in desparate need of work (standing around with nothing better to do).