The best way to do it is to get a donor car with DI. There are various sensors you need to change, then you'll need the wiring harness and DI/APC controller. Then of course there's the fitting. There are a couple of american websites that have this documented on them, but I haven't got the links to hand.
I believe that for one year only, the 9000 had a separate DI controller that can be fitted without worrying about the APC (DI & APC were combined on later cars). Eric van Spelde is the man to speak to about this, as he is fitting this system to his race-tuned C900.