There's a couple of ways of going about it...
1) the cheap way...
You plug the USB modem into your primary computer. You then plug your computer into your router where you would normally plug in a cable modem, DSL modem or some other WAN output connection. You would then need to reconfigure your computer to become the gateway computer (i.e. all internet traffic comes in and out of that computer. Of course, that computer would then need to be on for any other computers on the network to communicate with the outside world.
2) the not so cheap way...
Buy one of these: - or some other Cradlepoint product that does a similar job.
http://www.wirelessnwifi.com/index.php?main_page=product_info&products_id=28&zenid=a2b54211cb00a37e2f889753264b804dCosts $99 + S&H
You can then plug the output of this into your existing network, or if you're wireless already ditch the router and use this as your access point.