Encrypt E-Mails that were imported

I have migrated to mailbox.org and using thunderbird I can export/import all my EMails.

However, I want to use the encrypted mailbox(using my own PGP-Key). Incoming mails are properly encrypted serverside and properly decrypted on thunderbird, but I can't seem to encrypt my imported mails. I've read that I should just move the mails to the sent directory, that doesn't seem to trigger the encryption.

Is there a mailbox.org way to encrypt old unencrypted mails?

If not that would be a big problem, do you know a way to do that reliably for my old mails?