To import the OpenAPI definition in the NWC login to your NWC tenancy and open the Xtensions section. You can add a new custom connector there. Select the exported OpenAPI file and click on Next.
In the Security section APIKey is preselected as authorization method. You can leave the standard values for Name and Location and click on Next.
After that enter a name, description and choose an icon for your custom connector. Then you can publish it.
After publishing the custom connector will be automatically available in the NWC workflow designer.