Hi there,
have a look at the thread in the list with subject
Ticket #1484558 - Patch to allow a user to change passwords
where there has been some discussion about this. The Trac bug/wishlist entry is http://trac.roundcube.net/ticket/1484558, and you can see that it has been consider a plugin add-on that will be implemented at some time later :)
Cheers,