I'm not positive, but I think that you're confused. I think the product you linked to allows you to use a threadless CK headset as a threaded one, and/or vice versa. It converts your existing *headset*, not the interface.

There is a threaded-to-threadless stem adaptor. It is a quill shaft with a blank to which you clamp a threadless stem.

An adaptor to allow the use of a quill stem with a threadless headset strikes me as silly and difficult. It would only work with 1" forks, which are increasingly rare. It would require somebody to preload the headset bearings, clamp *something* to the steerertube that's *not* the stem, and then you'd have to remove the starfangled nut and insert a quil stem, hoping that the internal diameter of your threadless fork is correct.

It kind of boggles the mind.