Can a threaded headeset be converted to a threadless headset
Not sure why you would attempt to use a threaded headset as a threadless headset - other than trying to save on something which you can't.

The stem and top-cap assembly of a threadless headset are integral to securing the fork into the frame. Whereas a threaded headset secures the to the fork using a large nut.

There is nothing to stop you buying retro one inch fork, threadless headset and stem. That would be the safest way to do it.