I got this working too along with a switch. This is what I did.
After a lot of research and making mistakes I have finally solved the subject of this post and thought I would share it with you.
Previously, I had set the Superhub into Modem Mode and this did enable me to use the Apple Extreme 's WiFi instead of the Superhub's which gets turned off in Modem Mode. Modem Mode also turns off three of the Ethernet ports as well.
So when I plugged an Ethernet cable from the Apple Extreme into a switch the switch would not work.
The correct way to achieve the subject of this post is as follows:
If in Modem Mode as I was reset the Superhub and then turn it off and then back on again. After a few minutes the lights will come back on including the 2G and 5G lights.
Now I have WiFi from the Superhub.
Then using a short Ethernet cable I plugged one end into the switch and the other end into the Superhub.
To test that the switch was working, I turned WiFi off on my Macbook Pro and plugged an Ethernet cable into the switch and into the MBP. It worked.
Having determined that the switch now worked I was able to use the switch for my TV, DVD player, TIVO, etc
Now my challenge was to extend the WiFi of my Superhub using the Apple Airport Extreme.
So using a paper clip I reset the AE.
I ran an Ethernet cable from the AE to the Superhub and not the switch.
On my Macbook Pro I went into Airport Utility and found the Extreme on the button labelled Other WiFi devices. Click on that button.
The Extreme will be found there. Select it.
It will invite you to set up the AE
Click on the Options button and select Join an Existing Network.
Replace where is says Apple Extreme and type in the WiFi name of your Superhub - the 5G name.
Type in the encryption key of the 5G network.
Click Next and the AE will extend your Superhub's WiFi signal.
Virgin Media with Superhub 2 ac