This reply appears to address my question:
https://www.mobileread.com/forums/sh...postcount=2147
Quote:
You add folders using your PC ... For use with calibre and koreader, I would make it a folder inside a hidden folder. (Perhaps; .koreader/inbox, then long press on "inbox", in File Manager, and set it as your inbox folder, when asked.)
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That, I presume, predates the changes to the file layout - i.e. .koreader/ is now .adds/koreader. I suspect that .adds/inbox is probably a good choice?