So the bike originally had a rigid fork, and you want to put a suspension fork on it? If that's the case, you'll probably be fine as far as the bike's handling, just don't put a fork on it with anything more than, say, 80mm of travel. Even then, your bike may be a little less responsive in the handling department, but will likely be okay. I've done this type of conversion before, and it worked out fine. And as Little Darwin said, as long as your bike's headtube is the right size for the fork's steerer, get an appropriately sized threadless headset and install it so you can install the threadless fork. For details on installing the headset and cutting the fork steerer to length, etc., go to the Park Tool website, click on "Repair Help" and go from there. If you lack the tools or confidence to do it on your own, you could get the whole conversion done (labor) at your LBS for not all that much money I would think. Good luck-