Dial-up and Broadband connections coexist in harmony

I am running a server 2003 behind a heavily built firewall. The server mainly
provides FTP access within my department. The only ways to connect to the
internet is via localhost ssh connection to the organization's unix server,
and tunneling to public proxy servers out there in the real world; or use a
dial-up connection. Our colleagues want to get access to the FTP server at
home. How can I configure the server listen the request from the dial-up
connection and route all the out going packets through localhost to the
internet, at the same time maintain the normal FTP service within LAN
connection? I also don’t want to see the dial-up connection take over my
internet connection.