if you have amodem router there would be no point... you either want a router with ADSL modem built in or an ethernet modem and a cable router. There are few router out there that take USB modems and it's usually cheper to get a router with modem built in.
What type of USB modem? 3G? The Billion 7402NX et al have USB ports for this purpose (either to share a 3G connection over the LAN wifi, or to use as failover for ADSL).