The process is the following:
- In Mail’s File menu choose Import mailboxes
- In the dialog that appears select Files in “mbox” format and click Continue.
- In the open file dialog select the folder with the extension .mbox created previously.
- The program imports and recreates all previously archived emails.
The new folder is called mbox and it will be inside a new folder called Import. You have to rename it and place it where it interests you.
With this simple process you will have imported all the contents of the folder that you have previously exported.