Using the same IP and port on one server, you can't. The FTP server software must open a TCP socket in "Listen" mode, and only one listener can be on any address/port combination.
You can, however, buy FTP server software that will direct incoming users to a specific directory. The Microsoft FTP server doesn't do this (at least under my copy of NT 4.0).
We have such at work called FTP-MAX 2000, which is rather interesting -- it uses a MS SQL database to store the FTP site and control access. It's only drawback is that it can take up to 60 seconds for a new file to appear in it's listings (that's it's refresh rate).
Chip H.