part of your question was sort of covered in this thread: http://www.bikeforums.net/showthread.php?t=378330

As far as I remember, the Shimano FF system had a rear cassette that isn't truly fixed. So you can't just use the same cassette and wrap the chain around the existing cassette. The rear was a "friction freewheel," as it had a much higher friction threshold than the front, but would let loose if the front got caught up (with laces, pant legs, or a limb).

I'm not completely sure, but I seem to remember many of the hubs being proprietary to the FFS system, meaning that you could only use the specific FFS cassettes, and couldn't just thread on a cog. It may work, but I distinctly remember some hubs being proprietary for the Front Freewheel system.