The i5/OS Information Center provides a wealth of information about using your IBM system. From here you can select the level of operating system you are interested in (V5R3) and from the V5R3 Information Center you can enter ‘secure ftp’ as a search arguement. After this you will see Scenario: Secure FTP with SSL. Selecting this will give you instructions on creating a secure FTP environment.
There is much, much more available also starting with FTP.
I hope this helps you out,