I don't think you can.
The purpose of the Inbox as I understand it is to sequester new tracks for review before adding them to the library. I don't think it's meant as a permanent categorization or to delineate different sections of a library.
Search works because that's a useful tool to review the inbox tracks side by side with library tracks to be able to copy tags, check for duplicates, etc.
Maybe adding the inbox tracks to the library but moving them to a different, temporary directory would serve the purpose you're trying to achieve with this?