Years ago when threadless steerers and headsets were first introduced, Shimano didn't make (and still doesn't make) any threadless headsets but their sponsored Pro teams wanted to use threadless forks. The team mechanics lathe turned the adjustable cups of Dura Ace headsets to remove the threads and allow their use on threadless steerers. It had to be done carefully to keep the race concentric and a very precise fit to the steerer tube but it apparently worked. This is not a job for a file used by hand.