Moving individual Mailbox to external drive
Q: Do you want this archive to show up in mail, or just be available should you need it?

If you just want to store the messages "offline" but have them around if needed at a later date you can select the mailbox in Mail, then select Mailbox -> Export Mailbox... (make sure you select "Export all subfolders") and save the Archive. Then you can delete the mailbox from mail and pulpit back in when needed.