Can I connect the Smarthings ADT hub with multiple Smartthings Wifi hubs to build a mesh network of Smarthings ADT sensors?
Can that mesh network contain multiple Smartthings ADT hubs so I could, for example, have a panel at each doorway?
No. The ADT hub is simply a SmartThings hub with additional security features. You would still need to have a Wifi router (mesh or otherwise).
As for multiple ADT hubs, the answer would be no as each hub, like the SmartThings hub is tied to a location and devices can only be attached to a single hub at any time.
Thanks for responding.
Unfortunately I am not following you answer.
As I understand it, the Smartthings Wifi hubs act as both a mesh Wifi router and a mesh Smarthings hub. That is, if we ignore the Smartthings ADT hub for a moment, with a set of Smarthings Wifi hubs, I can extend the reach of both my Wifi network over what I would get with a normal router, as well as extend the reach of my Zigbee/Z-Wave network over what I would get with a single normal Smartthings hub.
Is that your understanding as well?
If so, then what I would like to know is can I include the Smartthings ADT sensors as well as the Smartthings ADT hub in the network so that I can subscribe to the ADT monitored security service and for example have the police called if motion detector goes off when the system is armed.
With the Smartthings ADT hub and the Smartthings ADT sensors I get the monitored service. What I am hoping is that with the Smartthings Wifi hubs I can extend the reach of that network so I can include ADT sensors at more distant locations.
Make sense? Let me know what you think.
Unfortunately, No. 
Only the hubs which are designated as part of the SmartThings Wi-Fi router model line can be used together. They have special software that builds the Wi-Fi mesh. You cannot use any of the other SmartThings Hubs, including the ADT /SmartThings Hub.
In addition, the ADT/SmartThings dual logo sensors use a proprietary frequency. Only the ADT model line has the radio for that frequency, so the sensors must be in range of that model hub.