Unfortunately, it’s not just about the voltage, it’s about the waveform. So you can’t do the kind of science project thing you were talking about.
The shortest answer is that you’re not going to find a Z wave solution for what you’re looking for.
It’s possible you might be able to find a fan with variable control that has an open API or an IFTTT channel that. you could get in direct integration that way.
Another alternative would be to see if harmony works with your fan. If it does, you can set up integration with smartthings using a harmony activity. But again it’s probably not going to be continuously variable speed. But it might offer you more presets.
I know that’s not anybody’s favorite answer. It just is what it is.