Yep, everything needs to be an NSGroup.
Is the NSX Manager talking to (configured as a Compute Manager) the management vCenter?
What are you trying to add it to? NSX Data Center attempts to enforce traffic in separate stages (at source vNIC, at destination vNIC) - I don't specifically recall being unable to use external VMs in inventory objects but it's not outside the realm of possibility that this changed to ensure enforcement is possible.
This doc seems to infer that EAM (ESX Agent Monitor) is responsible for adding VMs to inventory on a host-by-host basis: https://docs.vmware.com/en/VMware-NSX-T-Data-Center/3.2/administration/GUID-79749A9D-4DC7-4C24-B667-...
My best guess is that you need to add as an external object to avoid implying that because a VM object is present, it can be enforced on with both stages.