If I get you right, you wan't to be able to ftp to the xbox over the internet.
You will need a router to do this. In this router, you will need to forward the ftp port (21 if I remember well) to the local ip adress of the xbox.
You will need to ftp to the internet ip adress of the connection, or you wil need a service like dyndns or no_ip to give it a name on the net.
The router will forward the ftp request to the xbox, and it will work the same as a normal ftp from your local pc on the network.
regards.
