.
...see if you can get someone to move this to C+V or Mechanics.
I don't think any of these guys use freewheels any more in road.

There are different cogs, made of different stuff. I've never tried welding them.
Most of them can be welded using proper material and technique, but I'm not sure about warping with the heat and cooling.


I honestly cannot picture what you're proposing to do, and cannot imagine it would not be easier to just get a freewheel with an extra gear.
I think they make them all the way up to 8 cogs now, but most of mine are 7 or less.