unfortunately there is no easy way to set this option. We are currently working on an opportunity to add a custom main.cf.local config (see here).
In the mean time you can add your option to one of the template files for the
/etc/univention/templates/files/etc/postfix/main.cf.d/. My choice would be the
10_general. But pay attention in updates of this template. Check after UCS updates (especially in postfix/mail updates) the templates e.g. with