I asked in the Calibre forum and found a plugboard solution, many thanks chaley:
https://www.mobileread.com/forums/sho...d.php?t=202802
In Calibre go to Preferences-->Metadata plugboards and add a new plugboard:
Format: any format
Device: KOBOTOUCH
Source template: {authors:'re(re($, '(.*) & ', '\1 and '), ' & ', ', ')'}
Destination field: authors
Then just enter all authors as normal, seperated by ampersands in the author(s) field, and the Kobo will display them in the same way it does for kepubs:
i.e. In calibre enter: Walter Jon Williams & Steven R. Boyett & Conrad Williams
Kobo will display: Walter Jon Williams, Steven R. Boyett and Conrad Williams