I tried importing a CSV address book to Claws Mail. It turned out Claws Mail doesn't support CSV/LDIF address book imports. This is a must have for anyone that wants to switch over from an old MUA to Claws Mail.
LDIF is supported (Addressbook/Tools/LDIF)
Oops, I meant "CSV and vCard import". Both are popular schemas for importing and exporting address books.
vCard is supported (Book/New vCard)
You are right. This way works. But I found this counter-intuitive. I would have expected that vCard entry (additionally?) at (German Version) "Addressbuch -> Extras -> Import vCard file... " What about giving the GUI such an extra entry?