openntp not really a good choice as ntp server imho. Not good way to monitor what its doing with ntpq or ntpdc, etc. like you can with the full ntp client. Its ok for keep some boxes time somewhat correct.
If you want to join the pool, just run full ntp on your pfsense box. Simple enough to do. This is what I did before I moved to virtual, now I use ntp on the actual host hardware as the ntp server vs the pfsense box.