I want to connect my old eMails from Opera as Mbox-file(s) with read only.

Opera can export the inbox and outbox separately. As import, Opera needs bothe files separately - what about KMail? Should I use the Mbox-file within incoming and outgoing messages or two Mbox-files?

Are there any restrictions to the search in Mbox-files (read only)? I discovered non so far in my tests.

I think it should work OK in principle. I haven't used mbox files in years, but I don't see why your approach wouldn't work.

Thanks.

...but Inbox and Outbox in one file or in separate files? I don't want to mess up my eMail-archive.

I'd keep things separate.