the newer model has an IR flash option, I assume you are trying to avoid the upgrade cost? You might find that selling your current one on ebay and buying the new one an easier approach.
Hmmm, it's possible but you'd need to determine some things. Does the IR illuminator output enough IR to take a picture? Does your camera have an IR filter integral to the lens (many do)? You should test that - take a picture in the dark with illuminator on. its should be obvious.
I don't know how the illuminator ramp up time compares to the focus time of the camera. It may be a wash but even so, I'd not go for the flash output as it seems difficult to harness the voltage to the xenon strobe. At the very least, I'd find the trigger to the strobe and use that. but...
The camera has LEDs to indicate motion activity. Do these typically come on before the picture is taken? If so, instead of hooking into the flash system, I'd hook into the LED circuit. Use this to turn on the power to the IR illuminator. You should definitely power the illuminator seperately from the camera. I'd use a logic level N-MOSFET to control power to illuminator though you may need to use a transistor to supply a higher voltage to trigger the MOSFET. You might also want put a variable delay into the circuit if the LEDs come on a lot with out the camera taking a picture.
edit: wow, in the 15 minutes it took to compose this you guys had quite a chat... also, I thought about the seperate motion sensor but that may be tricky to set up so both it and the camera sense the same location/activity.