Without knowing how your network is set up, I can only offer the following:
You mentioned a NAT - which I presume means that you want to map an external ip to an internal (does this mean you have a dedicated external (internet) ip or are you just do a NAT to a WAN? This is normally taken care of at the firewall, rather than the DNS server, but again, I am not sure how your network is set up.
On our setup, we just set up a DNS Host record in the Forward Lookup that maps the DNS name to the internal ip.
Everything else is handled at the firewall (which takes care of the NAT) and the domain host (which has another Host record pointing the DNS name to the External IP).
