[abrogard]

would like to be able to add tags outside of Calibre, before I import to Calibre and I have been trying to use exiftool but without success.

the tool thinks it has succeeded but after import calibre sees no added tag.

should it be/is it possible to do this?

I'm talking a custom tag here, like we do so easily within Calibre. Like maybe 'booksfromjim', that sort of thing