An NSX for vSphere environment can contain hosts that have NSX for vSphere installed, but are not added to a vSphere Distributed Switch. You must add the hosts to a vSphere Distributed Switch before you can migrate them.
You can use a distributed switch you already have in your environment, or create a new distributed switch for this purpose. Right click the distributed switch and select Add and Manage Hosts to add the hosts to the distributed switch. You do not need to assign physical uplinks or VMkernel network adapters to the distributed switch.
See "Add Hosts to a vSphere Distributed Switch" in the vSphere Networking Guide for more information.
If you import the configuration before you make this change, you must restart the migration to import the updated configuration. See Make Changes to the NSX for vSphere Environment.
After the migration has finished, the hosts are no longer required to be attached to the distributed switch.
If you added the hosts to an existing distributed switch, you can remove them from the distributed switch.
If you added the hosts to a new distributed switch that you are not using for another purpose, you can delete the distributed switch.