MX stems are round with a "+" in the middle.
Can you drill out the "+" to make it "O", then cut a notch in the top and bottom to turn it into Topre compatible?
You probably could, but it wouldn't be stable enough to support constant typing.
doesn't work at all, actually
the stem on MX caps is too small
You lot are a bunch of wet blankets )
So I went ahead and tried anyway.
The MX stem is about the same width as the Topre stem.
The hole in the middle of the Topre stem is about 5/32" (or that is the drill bit that most tightly fitted).
I drilled out the middle of a couple of MX keycaps from a Crap Bag - no big deal.
The problem was carving the slot in the upper and lower "edges" of the stem. On Topre this is narrower at the end of the stem and widens as you go up towards the typing surface of the keycap. Careful inspection of the Topre slider reveals a triangular bit that presumably the stem clips over to hold the keycaps on.
I carved a slot and the keycap went on, but wouldn't stay on.
I tried to carve it wider at the top of the keycap and narrower at the bottom, but that was kinda awkward.
One attempt sort of clipped on at the bottom of the keycap, but not the top.
The other one sort of went on and stayed on, but was really loose.
Also the MX stems are not as long as the Topre stems. The Topre ones extend out past the bottom of the skirt.
So I guess with a bit of care, some more time, and some proper tools, it would be possible. More or less.