The NetDefend firewalls have been ICSA certified for IPsec-Enhanced and VPNC certified for Basic-Interop and AES Interop.
This means you should be able to use them with any other VPN endpoints that are either certified themselves or are designed to be compliant (regardless of certification).
More specifically I have personally seen OpenVPN tunnels to a DFL-800, though I can't claim to have the OpenVPN config file, nor as much as the version that was used.
The DFL-800 config is the same as any other IPsec configuration, the OpenVPN half, I can't help you with.