Yes, you can do that. However, I believe that configuring the AX over Ethernet requires using Airport Admin Utility instead of Airport Setup Assistant. What you'll want to do once you get to the Airport Admin Utility configuration panel is to turn off wireless in the Airport tab (assuming that you don't want any wireless access; if you later obtain some wireless devices, you could enable wireless on the Express and use it as a wireless access point for your wired network) and disable "Distribute IP addresses" in the Network tab. At that point, the Express will be just another peer on your Ethernet network.