we dont have command line utility for Nintex Forms product. However, what you could do :
1- Nintex Forms for List : stored in the hidden library : http:/<SiteURL>/NintexForms/Forms/AllItems.aspx
So you can download the XML file for the required List Form.
2- Nintex Forms for workflow Task actions : stored in the Nintex Workflow definition file .NWF it self. so you would need to use the UI export/import button.
Hope that help.