It is possible to use NWAdmin.exe to export workflows.
But, is here another NWAdmin.exe command to import and publish a workflow ?
I don't find that in the documentation... 😞
While NWAdmin.exe can export a workflow (using the ExportWorkflow operation) to an export file, it cannot import nor publish workflows from an export file.
Alternately, you can use the SaveFromNWF* and PublishFromNWF* SOAP service operations in Nintex Workflow to import and publish workflows from an export file. For more info, see Saving and publishing workflows, in the Nintex Workflow 2013 SDK.