I don't think the Apple USB modem works with Time Capsule, but I would love to be proven wrong.
One way to accomplish this would be to have a computer with a dial-up modem to be the main connection point, and then share that connection over ethernet to the Time Capsule.
Another method that I have used is to use one of the very old AirPort Extreme base stations with the built-in modem (the UFO shaped kind). They can be found on eBay for pretty cheap. I used it for G wireless and connected a previous model Time Capsule to it via ethernet and used the TC for N wireless.