The default configuration of Dovecot already includes the ability to add custom configuration options for Dovecot that are not already covered by the templates in /etc/univention/templates/files/etc/dovecot/*and the corresponding UCR-variables.
root@imapserver:/etc/dovecot# tail -1 dovecot.conf
!include_try local.conf
Any valid configuration option (see Dovecot Config File Syntax) added to /etc/dovecot/local.conf will become active when restarting Dovecot. In case an option was defined earlier in the configuration chain the settings from local.conf will override it.