Trying to build a bike, confused about fork/headsets/compatability
I would get a fork with a steerer that fits the frame. The expense and risk of modifying your existing fork won't be worth it. There were some mountain bike forks with replacement steerer tubes but they didn't allow for changes of size or type of steerer. They were 1 1/8" threadless and so was the replacement. You also want to match the fork rake/angle that the frame maker designed into the frame.