"UDP Offload Example" to "Point to Point Ethernet System"

Basically you have to make your own component before the DEMUX that finds out if an incoming packet is destined to the Nios system or your custom module, and give the correct channel number depending on that.

As you are using raw ethernet, one way to do that could be to use a special Ethertype value for your packets. Then you can use a component similar to the UDP port to channel mapper, but that would read the Ethertype instead of the UDP port number. If it is your custom Ethertypem, redirect the packet to your custom component, and if not redirect it to the Nios system.