We are in the process of migrating our IS v6.1 from HP-UX to AIX hardware.
The pub.client:ftp services use the custom com.wm.net:FTPSession class, which is built around the standard java.net API. It does not wrap any OS FTP client and therefore cannot be replaced by another implementation.
If you want anything more elaborate than the standard functionality you would have to write your own services, though I would recommend basing them on a third-party Java-based solution, which you can build Java service around, in preference to wrapping an OS executable.
Software AG Professional Services