@WildEnte
Thanks for the answer!
That's what I was thinking when reading about it (after getting myself in a very bad situation, due to all contacts lost 😞 ).
I saw that Thunderbird had a few extensions for this use-case.
I was thinking: Install Thunderbird -> get all mails -> create contacts based on mails -> export contacts -> import into Vivaldi
But now it seems even contacts can't be mass imported.
The feature seems so close, but yet so unreachable 😊
I was also thinking about using SQLite directly and modifiying the DB file (Contacts), but it's not documented, and I would still have to get the list of addresses first 😩