Note that disc brakes place forces on the fork & frame which are very different from rim brakes. If you try this, best to use a heavy, overbuilt frame to avoid chatter, flex, and possible failure.

http://www.novacycles.com/catalog/pr...roducts_id=811

If you just want front disc brakes, just swap out the fork for one of the dimension disc-only forks. 1" and 1 1/8" size threadless, might be possible to thread them to fit an older setup.