Because a threadless stem also functions as the topmost piece of the headset, setting bearing preload (or just eliminating play), it's not just a matter of sticking a quill stem into the steerer tube. Plus, a threadless fork's steerer might not be built for the stresses a quill stem wedge or expander plug might exert.

One thing that should work, if you find it aesthetically pleasing enough, is a "classic style" threadless stem like the ones Velo Orange carries:
VO Tall-Stack Stem 31.8, 1-1/8" - Threadless Stems - Stems - Components