The most recent update to Kea 3.0.2 also brought the ability to export/import DHCPv4 reservations to a csv via the web interface. While this is a welcome addition, sadly there is no analogous buttons in the Kea DHCPv6 reservation screen.
I hope this was an oversight/bug.
Ticket on GitHub would help if that's the case: https://github.com/opnsense/core/issues/new?template=feature_request.md
Cheers,
Franco