Generally, cutting threads in a threadless fork is thought to be a very bad idea. The wall thickness of a threadless fork's steerer is the same as the thinnest part of the steerer of a threaded fork. So cutting threads in would make the walls thinner than is generally considered to be safe.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"