You must deploy a new NSX Manager appliance to run the migration coordinator. Do not deploy an NSX Global Manager.
In other words, you cannot merge your NSX-V environment into an existing NSX-T environment, which has NSX-T already installed on the vSphere host clusters.
For details on deploying a licensed version of the NSX Manager appliance, see Install NSX Manager and Available Appliances in the NSX-T Data Center Installation Guide.
Install one appliance to perform the migration. Deploy additional appliances to form a cluster after the migration is finished. See Finish Deploying the NSX Manager Cluster.
If you install the NSX Manager appliance on an ESXi host that is a part of the NSX-V environment that is migrating, do not attach the appliance interfaces to an NSX-V logical switch. To prevent the management VMs in NSX-T from losing connectivity after the VMs are rebooted post migration, tag the management VMs. For more information, see Tag Management VMs in a Collapsed Cluster Environment.