Yup, adding to the pile-on here. Going from threadless to quill involves more than just trimming the steerer tube. You will have to replace the headset, with a threaded topnut, and extensively modify your fork steerer tube, if not replace it entirely, since most modern (threadless) forks aren’t intended for use with quill stems and threaded headsets.

If this is to solve a fit issue, threadless is the much better way to go. Threadless stems come in a much wider range of lengths and angles, to better dial in your fit, and nearly all have removable faceplates, allowing you to swap stems without having to dismantle your handlebar grips and controls, like you do on the pinch-bolt clamps found on more common quill stems.