Personally I would install a second nic on the fog server. Have a dedicated imaging network and then a business network management interface. This is a normal configuration for FOG. Your FOG server will supply dhcp for the imaging network only.
You can change the IP address after FOG is install, but its a bit of a pain. All you need to install FOG cleanly with dual network interfaces is to know the interface name of the imaging network and to have your business network nic to be able to get to the internet directly or via a proxy server.