Right, so you have a USB modem which obtains an internet connection wirelessly through the 3G network, and you want to connect it to your router, to share wirelessly with the other computers in your house?
If it doesn't have an RJ-45 (network) out, then I can't think of any way this is possible. Possibly the computer it's plugged in to can share the internet connection, but I know this is far from convenient, especially if you have a laptop and don't want to restrict yourself.
You could, however, fork out for a Wireless USB hub, and dongles for each computer:
Belkin Wireless USB Hub. Unfortunately it's about $200, although that's a third the price of the ISPs router...
EDIT:
If you have a spare computer, what about plugging the modem into it, and the computer into the router. Then it could just sit there as a gateway. Doesn't have to be a good computer, an old one would do.