applications need to be tweaked to conform to the Carbon API where about a third of the APIs in Classic have been removed. so a recompile is the least needed - and for that you need access to source code. If it were that easy, people would have done it. (long answer)
Hmm, okay. I was bored one afternoon and I tried to make classic AIM be identical to the Mac OS X version, but it didn't work. Somethin with the libraries or something. I'll just wait til the companies come out with OS X versions of the apps.
If they invent those converters, they could go straight ahead and invent a water-2-wine thing...or trash-2-cash and stuff like that...for a company which achieves the first goal, the last two thigns should be easy