Make permission system for changing vCard

Hi
I need to develop service or plugin for ejabberd, which forbid user change his vCard.
User register in system. Initially he can change vCard. But administrator can set up him prohibition to change some fields in vCard.
How i can do it? I don't want to make changes in ejabberd. I'd like to update server in future.
Can i write plugin/module for ejabberd without changing core source code?