PGP - Encrypt imported emails?

I have enabled mailGuard and Inbox encryption, both for my standard mailbox ´.org address and my custom domain. Which is nice. But I have yet not imported or moved my emails from my previous provider into mailbox. Because I can't figure out how to encrypt them, if I move or copy (drag and drop) them in my email client, from my previous inbox to mailbox.org inbox, they don't get encrypted as I understand because they don't trigger the Sieve filter. They bypass the filter and ends up unencrypted.

Is there a way to import unencrypted emails into mailbox.org as encrypted?