Ok, so, that specific scenario won’t work on the ST side, the ST Schema will have access only to those devices your connector has defined to discover.
The POST call of the callback URL won’t be triggered again until the connector is completely deleted and added again.
Instead, you should define a UI on your cloud to allow users to add more devices to SmartThings and you would need to send a discoveryCallback.
The same happens if you need to delete a single device, if it’s deleted on the ST side, your cloud won’t get a notification, so, the user needs to indicate on your side he wants to remove it from ST so you can send a stateCallback with the corresponding error message.