Yes it can be done, the early Tuff II had steel front hubs & steel rear hubs with a cassette type freewheel, I had some of these & the freewheel was not working so I removed it & fitted a Suntour coaster break, so the shell is the same, its just finding one of these early cassette freewheels is the problem
Your other option would be to fit a freecoaster freewheel which you can get, its made by ACS, I may even have one somewhere
http://www.vert-ego.com/main.htmClick on the Freewheel / Coaster adapters link & its at the bottom.
Or removed the hub & fit some replacement Alloy hubs which take a screw on freewheel.