Yes, you can, but we've seen a lot of confusion regarding OpenDNS - many of our customers want to use OpenDNS as a "second layer of protection." While this is all well and good, most of the time we see people putting OpenDNS's servers on NG Firewall's external interface, which is not the right way of going about it.
We always recommend using your ISP's DNS servers on any WAN interfaces of NG Firewall. We do not recommend using OpenDNS, public, or internal DNS servers as they can hamper the effectiveness of Spam Blocker.
If you want to use OpenDNS with NG Firewall you should hand out OpenDNS as the DNS servers for the end users only. To do this, you'll need to go to Config > Network > Advanced > DHCP & DNS, enable it, and enter the following into the box:
This way, NG Firewall will hand out OpenDNS to the clients it gives DHCP addresses. If you're running your own DHCP server, you'll need to figure out how to make the change for your particular server software.