there are quill to threadless conversion plugs, to grip a threadless stem around,
and still use the threaded headset around the fork.

and a tube type stem raiser, with a quill at the bottom, reduced section.
tube is formed so as to have the OD at the bottom, and the ID at the top, the same.

and the OD at the top the same as the fork OD at the bottom.

If you wanted to use a threadless headset, a compression plug at the top,
would come out to get to the quill bolt, and operate as the removable substitute for a star nut.

BBB has a most clever combination for their stem raiser. long quill of aluminum,
a bolt to pull the wedge tight that is made to be internally threaded at the top,
a bolt threads into it to, for the top cap for adjusting threadless headsets,
and a stack od spacers with a key tab, that engage a keyway extruded in the stem quill.

I got one of those [BHP-21] to bring the height up, on my Trekking bike,
enabling a second stem to be fitted, beneath its Trekking handlebars,
for my handlebar bag's KF QR Mount.