Use current Toolbox command for importing OAS and add an option to target a backend API (instead of a Product). The OAS itself won't be stored in 3scale but a backend-api, private base URL, mapping rules and methods will be created.

`3scale import openapi -d <remote> --backend <OAS>`

• behaviour when no private base-url (no server object): return understandable error.
• some existing options don't make sense at backend level: document.