I have a setup using postfix with postfixadmin for managing virtual mailboxes and dovecot for imap and sasl authentication.
How can I set up the password plugin allow users to update their passwords through rouncube?
I don't think there is, yet, a specific driver for postfixadmin to change the password via postfixadmin command line.
However, check the README for the password plugin at section 2.1. Database (sql).
You can follow the instructions there to change the password directly in the postfixadmin database.