FOP2 can have static or dynamic configurations, as such there is no way to have a "universal" plugin for users to change their passwords. If you usee FreePBX/FOP2Admin you can update passwords updating a mysql table, but if you use manual configuration that won't work, or if you use a custom autoconfiguration, it won't work either.

It is not that hard to write an utility to modify passwords from a mysql table, but if you do not store it in mysql it won't work at all.