The community plugins for the mainstream release of FreeNAS 11.3 has a community plugin repository. It does feature the Unifi Controller version 5. If you’re looking for the Unifi 6.x code stream you’ll need to build a jail manually. Below you will find the quick guide to manually setup a Unifi6 jail in BSD.
pkg install mongodb36 snappyjava portsnap fetch portsnap extract Mk portsnap extract UIDs portsnap extract GIDs portsnap extract Templates portsnap extract net-mgmt/unifi6 cd /usr/ports/net-mgmt/unifi6 make install clean echo 'unifi_enable="YES"' > /etc/rc.conf service unifi start