You obviously don't know my plugins.
They are a collection of 9 plugins where the account plugin is the base for all other 8 plugins.
So if you want that your users can change their password in roundcube, you would to activate the ispconfig3_account and ispconfig3_pass plugin.
Of course there are also other plugins like autoreply, forward, etc
In roundcube the plugins will add a section under settings called account.
You can find all information like install instructions, etc and the plugins itself