As long as the AS400 has a working default route this will work. You just need to get an external IP address from your ISP and setup a Mapped IP address in your firewall. The FTP server will be able to access the AS400 using the external IP address.
If the AS/400 is the client then you will need to run ftp from a command line to connect to the remote host. As long as the client has a route to that remote host and the firewall(s) is(are) opened for ftp protocol, you should be able to communicate. Here’s a FAQ about AS/400 FTP.