Cutting threads from scratch on a threadless fork is a whole different kettle of fish. It would be much more difficult to start the thread and to keep it aligned - unless the thread can be cut in a lathe, presumably somehow holding the lower end of the fork at the headstock end, and using a steady and/or a live centre on/in the steerer. This wouldn't be a trivial undertaking, and not inexpensive unless you could do it yourself.
Then you have the potential problem that the wall thickness of the steerer may not be designed to have a 22.2mm bore; the walls may be thicker to reduce the chances of breakage and warranty issues. It may or may not be feasible to ream or bore out the ID if this is the case, but would again add difficulty and expense if so.
Given the Soma fork is threadless then IMHO you should plan on using a threadless headset and stem. This will be much cheaper, much easier and more structurally sound as well.
There are plenty of things which could potentially cause grief here, and unless you enjoy bashing your head against the wall then IMHO "discretion is the better part of valour."
Later,
Stephen
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