You don't say what version of AIX you have, so things may have changed since I last managed one, but I think if you edit /etc/inetd.conf and adjust the line that starts the ftpd service, you can add the debug flags there.
The documentation for it is in man ftpd rather that the page for inetd or inetd.conf.
Sadly, I no have access to AIX, but I think it is the -d flag. You might also need to create an output file for syslogd to write to, add a record in /etc/syslog.conf to direct the output for daemon.debug to it and restart the syslogd (or perhaps kill -HUP it?)
I hope that this gets you started. Sorry I can't form the whole answer.
Does that make sense, or have I just confused you more?