If you want to move it (as apposed to starting a second one), then just go into /etc/services (InetD matches the service names in /etc/inetd.conf against the service names in /etc/services, to determine which port numbers to listen on.
SO, you woul end up with two lines like:
ftp-data 2120/tcp
ftp 2121/tcp
FTP uses two ports to communicate.