Takes a lot of heat to melt the brazing material in the fork crown to pull the fork steerer out,
but a skilled bike frame builder can , perhaps cut down the threaded steerer tube ,
sleeve the inside, then
tig weld a new piece of steel tubing on, and get it still, dead on, aligned.

Just Don't ask how Much will it cost..

Buying a threaded to threadless adapter is so much cheaper ..
http://www.bikeman.com/Stem_Column_Adaptors.html