IMHO, Phex should be able to do what you want just fine.

Is your network isolated or connected to the internet? If the latter (and if you don't want files shared externally), you may have to do some tweaking of the 'ignored hosts' settings.

Just run Phex (on the computer you're running the FTP server on now) and uncheck 'auto connect' on the 'gnutella net' tab. Then instuct people on your network to put the IP for that computer to the 'auto connect hosts' section. It will thus act as a host cache server even if all its sharing connections are full (then people can connect to other computers running a gnutella app).

Under 'Phex Options' -> 'Download/Sharing Settings -> 'Sharing', be sure to check the 'allow browsing of directories'

If you have any problems with this, let us know.