I want to create a new user for my Snow Leopard's built in FTP but I don't want to have to create a new account on OS X itself. How do I do that?
I don't think there's any way to; /etc/ftpusers will let you restrict which accounts can use FTP, but not add users beyond what the OS knows about. You can create a "Sharing Only" user in the Accounts preference pane, and then change their login shell to something valid (right-click or Control-click on the sharing user's entry in the Accounts list, choose Advanced Options from the shortcut menu, then change the Login shell to, say, /bin/bash). This user will be able to FTP in, but not log in to the desktop on your Mac. They will, however, be able to use other network services (other file sharing protocols, ssh, etc).
Sorry, that's the closest I can come to what you want.