If the shield has Smartthings installed via the dongle but you already have a ST hub can it be used as an extender?
I doubt it.
It’s just like using 2 separate SmartThings Hubs (which currently creates 2 Locations in SmartThings that happen to be under the same Account but can’t share the same instance of a SmartApp!).
Exactly what @tgauchat said, it’ll act as another entirely separate hub.