[Igor]

You can't modify ISPmanager
but you can extend it self

for example our filemanager made as extention.
i think we should have documentation about how to do extention

webmail, phpmyadmin and webalizer is just linked application
and you can link any another. for example script for install phpbb.

please see /usr/local/ispmgr/etc/config it should helps you to understand how extentions works.

also i'll ask to describe such tasks in "Step by step" forum