Freewheel bodies are HARD, and don't take kindly to machining, let alone thread cutting. IMO, you'd have some serious heat treatments to go through before being able to pull that off.

If I were to try for your goal, I think I'd start with a freewheel that'd fit. Then I'd consider two different options:
1) outside help - find a guy with a water jet cutter, and a good stock of sheet metal. Have him cut new sprockets that'd fit the existing spline pattern out of a suitable thick plate. Then cut spacers on a lathe, assemble and ride. Bevelling the tips of the teeth is easily enough, but you'd have to do w/o the ramps.
2) the home shop approach - Pull the sprockets off a couple of scrapped freewheels. Disassemble a 9-speed cassette. Use a hole saw in a drill press to cut out the splined center of the sprockets from both cassette and freewheel. Might want to use a slightly smaller hole saw when you cut the centers out of the cassette sprockets to get a slimmer seam. Fit cutout freewheel centers to cassette outer sections. Weld/braze them together depending on seam width, available equipment/skill, and personal preference. Cut spacers on lathe, assemble and ride.