Can this be done or do the threads on the steerer tube not allow it?

Yes, it can be done. I ran one for a year on my cross/commuter bike. It worked like a charm, despite repeated abuse and a good fork bending/wheel tacoing crash (not the reason i replaced it, i straightened the fork and used it for another 6 mo). This is assuming that the steerer tube is long enough and you install a star nut. I didn't do any of the wierd recomendations I heard like using epoxy to fill the threads, just bolted it straight on. The threading did 'scar' the inside surface of the the stem but it worked just fine when I later replaced the fork and never once have i had a problem with the stem spinning. The steerer tube of my fork was steel 1 1/8. Be cautious and make sure it holds before you decide to trust it, it has a drastically reduced contact area with the stem as compared to an un-threaded stem.

Can this be done or do the threads on the steerer tube not allow it?

As krashcountry said, you can do it if the steerer tube is long enough. That means that you CAN'T do it for an existing fork that is set up for your frame, because there is not enough steerer tube sticking out.

As others have said, it can be done but you'd need a fork for a very large frame to get enough steerer to stick out.

Is there any kind of adapter that will work? Seems to me I've heard of something like this, but I'm not sure. Anybody got any info re: this?

Is there any kind of adapter that will work? Seems to me I've heard of something like this, but I'm not sure. Anybody got any info re: this?

You can adapt the STEM to threadless with an insert unit that provides a clamping surface for the stem. However, a threadless HEADSET is another issue. I suppose that using the extender would allow you to slide the upper part of a threadless headset on there, but the whole thing would be a kludge, it would seem. I would want to see one in action before starting down that path.

Hmmm! I'm not sure I understand what you mean. By saying you can adapt the stem, do you mean that you could use a quill stem with a threadless set up?

There is an adapter that inserts into the standard threaded style fork and binds in place like a quill stem. It sticks out with a plain 1 1/4 cylindrical extention to allow the use of a threadless type 1 1/8 stem. For a threaded stem, one can not adapt to a threadless headset bearing assembly.

What is the goal of your inquiry?

For a threaded stem,

Oops, I meant to type for a threaded fork here.

There is an adapter that inserts into the standard threaded style fork and binds in place like a quill stem. It sticks out with a plain 1 1/4 cylindrical extention to allow the use of a threadless type 1 1/8 stem. For a threaded stem, one can not adapt to a threadless headset bearing assembly.

What is the goal of your inquiry?
I saw the OP's post & thought maybe I could help. My own inquiry? Just curious. Thanks for you explanation, though. Cleared it up for me. :)