Template for mailbox user names?

If the user creating a mailbox user, I’d like to make sure that the mailbox user has the main user’s name as a prefix.
I.e. if the main user is named “jo”, the mailbox users could be named “jo-private”, “jo-public”, “jo-mail1”, but not “private”, “public”, or “mail1”.

Is there a setting for such things? I have been looking in the virtual server templates and in various other places, but to no avail.