Sheldon used a quill stem in a similar configuration on his multi-handlebar...

http://sheldonbrown.org/thorn/index.html

A threadless system with a clamp (Sheldon used a seat post clamp on the bike listed above) can accomodate a threaded stem.

I would only try it with an alloy, or preferably steel, steering tube...

From what I understand a threadless steerer is the same as a threaded steerer except that...



(drum roll please)



... it doesn't have threads.