Originally Posted by Steev

I would not attempt this or have anyone else attempt it.
A fork designed to be threadless is very unlikely to have the wall thickness to support cutting a thread into it. The steerer may shear off either during threading or in service.
It will also be very difficult to cut such a large and deep thread by hand and obtain satisfactory results.
The die linked above is intended for chasing or maybe extending the existing threading on a fork. I doubt you could cut a full thread without dulling the die to uselessness.