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