You’ll have to cut the threadless steerer to drop the bars any appreciable amount. I’m not sure how you plan to pre-load the headset, clamp, and insert a quill stem on your now too short steerer.

As for threading it, and using a threaded headset, forget about it if the steerer is aluminum.

If you don’t want to get a different threadless stem that will drop the bars, which is the best (and only) route, get a separate threaded fork and go from there. No one will want that bike if you mod it for a quill stem.

John