yes, but its a little convuluted.
unless your router has a phone jack you'll have to do it this way.
Connect the modem cable from the phone jack to a spare pc's modem card port, just as you would normally, then you need to enable the computer as a gateway, you need a network card also which should then connect to the routers wan connection, and voila. If you need it more in depth let me know, haven't done this for 6 yrs.
obviously if you have an external modem, you can do without the pc, but the modem needs and rj45 jack.