Time Capsule WITHOUT Internet service

I have a new Time Capsule for my Mac. I set it up to do wireless backup and wireless connection to printer; all is ok. EXCEPT: my internet service comes from either one of two places (neither of which passes thru Time Capsule) - an internet card (Sprint) or a chance pickup of a local wireless network.

Problem: It appears I can't use Airport for two connections (time capsule AND local wireless network), and if I try to use internet card while airport is connected to Time Capsule, it says can't do multiple connections. Is my new Time Capsule a waste? I know I can fix this by buying service from Comcast, but I absolutely hate that option.

I'm probably not getting the scope of your problem, but couldn't you use the Sprint modem on your Mac, a wired connection between your Mac and the TC, and still let the TC communicate wireless with the printer?

Time Capsule is basically an AirPort Express router with a hard drive. This would be the same as buying any router in a store, and trying to connect to it while still in the box. Won't work, at least to my knowledge.

In theory, without having two airport cards, its impossible.