Opening maibox in Mail possible?

Hello,

so I collect my e-mails related to one problem into my EF folder in one mailbox together with other stuff. Now I need to check the e-mails and may be reply or resend some of them. Reply is easy, send again means I have to find the record, open it in Mail and send again. It works, but I was wondering if there is chance to open whole mailbox in Mail at once. It may be more convenient at some point. I tried to open it from Finder, but that fails. I thought these were standard mailboxes used by various mail programs and they should be useable in any of these programs?
Am I missing something?

You could select multiple messages in EagleFiler and double-click to open them in Mail.

That’s not possible.

They are, and Mail can import them, but it cannot directly open mailbox files. (Eudora can.)