What you would probably need to do is to configure the server in your office to divert any FTP access to a specific machine.
e.g., say your FTP files are on machine 192.168.0.5 then you would need to tell the server to allow FTP access and for the server to direct any access requests to machine 192.168.0.5.
You would then have to find out the IP address of the broadband (try click here) and then pass that information onto your colleagues.
One last thing. If the router has a built in firewall, you will need to allow the FTP requests to get through the firewall, i.e., you will have to configure the firewall to open the FTP port number and direct access to machine 192.168.0.5 only.
I am not an expert on these matters and I am sure there is someone else on the forum who will be able to give you a detailed step-by-step guide.