Just to check, I would try to slip a stem in the Soma steerer (if you can get your hand on one at the LBS). If it fits *correctly* it could be tapped with a piloted tool (as JD Thompson showed in your other thread).

Otherwise, yes it's possible. If your headtube is long enough the simplest would be to splice the steerer. Welding as Groovy Cycles does is probably the way to go but you could braze the inner sleeve too. To remove the entire steerer, a good way is to cut it above the crown race, then slice through to the brass line in a few sections and then heat them individually to peel them off. That would minimize the heat input. But that seems time consuming and will likely be costly... I would call some framebuilders to check. I suspect drilling it out would leave an oversized hole.