I raised a case in regards to the licencing aspect and this was the response back:
"I have checked this internally and The cause of this issue is a bug in the product. The license check is occurring before checking if the nodes/interfaces/volumes already exist in monitoring.
This means that the import fails if the number of objects in the monitoring and the objects in the scope of the discovery is larger than the total amount of allowed objects in
the license level"
So unless there is a change it looks like there isn't a way to use Network discovery to scan existing nodes for the purpose of importing missing resources. Unless you have a lot of spare licences...
Back to the drawing board to create a custom volume import process