Threadless top races aren't just laid over the steerer tube.The central bore isn't cylindrical but conical, with the pointy end downwards to take a compression ring, which is likewise conical. Then the spacers and stem comes down to push the compression ring into the top race, and simultaneously preload the headset.

If you want to do this, you'd still need to get a compression ring somewhere, and it'd be a lathe job, not a reamer, to get the conicity right.

I'd just use the complete top half of a threadless headset in the right dimensions instead.