Nighthawk X6 WiFi Router R7900 mesh w/ another ethernet wifi router?

That router isn't a mesh router and netgears mesh extenders don't function as a mesh when hardwired in. the smart roaming features are disabled in access point mode. You can add an extender and have it use the same ssid as the primary router but devices will tend to be sticky and stay connected to the router/extender they were connected to.