Place your custom shutdown commands in /etc/rc.shutdown, creating it if it doesn't exist.
The normal AIX shutdown script (/etc/shutdown) will execute /etc/rc.shutdown if it is exists and is executable.
Note that if rc.shutdown returns anything but 0, the shutdown will be aborted. If you want the shutdown to continue no matter what happens in your script, make the last command something that always returns 0, such ast /usr/bin/true.
Rod Knowlton
IBM Certified Advanced Technical Expert pSeries and AIX 5L