For my load testing, I have a need for my virtual users to connect to a ftpS server and put some files.
at this time I get this response 500 Illegal PORT command .
It can easily be done by loading the package FTPSClient instead of FTPClient in the link you provided.
So for instance use "myFTPClient =new Packages.org.apache.commons.net.ftp.FTPSClient();"
The same methods are working for FTPClient and FTPSClient.
For more information you can check that link