Originally Posted by
HillRider
...The wall thickness was not designed to accept threading and remain sufficiently strong... .
This is one of those cases where a measurement is worth a thousand opinions. Many (most) threadless forks have the same 1/16" wall steerer as threaded forks, and can be threaded. Rather than guess, it's easy enough to check. You can measure for a 7/8" ID, or simply stick the stem in and check for a firm fit. If the stem has a sloppy fit, the wall is too thin and the fork cannot be used for a threaded/quill set up.
Changing to threadless is a viable option, but means added expense for headset and stem.