You can get routers with for broadband primary use, that have a RS232 port (serial) for connection of a phone line modem or ISDN modem as backup.
http://www.d-link.co.uk/?go=jN7uAYLx...zk36riLU4Tt+g=
Dunno if there is a wireless equivalent - you could always just add a wireless access point to it. Gets expensive though.
And as others have said, the bandwidth off a 56k modem is so limited, I'd just sign for broadband now thus saving the expense and hassle of even seeing if it would work. The router needed can then be cheaper and a combined ADSL wireless device.