videocwk,
Interesting idea, though, I'm doubtful that it could be accomplished easily with satisfactory results. I don't think a sub in a 2.1 system has got the circuitry to handle a 4.1 or 5.1 system. In some checking with Klipsch tech support on similar questions, I found out that subs which might appear to be identical actually are not.
If you had a 2.1 system that you wanted to use as 4.1, it could be done with Y adapters, IMO. It would just be adding another pair of speakers for two channel use. I'd be careful, though, to not run it hard at all. Adding the speakers would lower the impedance presented to the amp. Depending on what the amp is designed to handle, it could cause the amp to fail.
As to the actual impedance of the satellites and the impedence capabilities of the amp, that would be an excellent question to have answered.
"I know I'm paranoid.... but, am I paranoid enough?" --Tom Clancy