Hi @Michi
In my humble opinion I think you will only be able to import into Thunderbird. This should be possible from the import menu within TB address book.
A faster way might be to use some plugins with TB which fetches the contacts from the Google Cloud of Google Contacts. This requires you to upload your contacts to Google. Not sure if you want to do that because it means privacy concerns.
See the initial statement regarding sync with KDE Connect from KDE: 456140