A confusing question. Do you want to do this just to change the type of stem the bike uses, or do you for some reason feel you actually need a threadless type of headset and fork (I can't imagine any reason for that, but I'm curious if you have one).

IF (big IF, since I don't know), all you want to do is be able to use a threadless type of stem, then all you need to do is remove the current stem and replace it with one of these:

http://www.nashbar.com/bikes/Product...0052_175545_-1___