Another way to import from Apple Mail is to drag and drop folders or mailboxes from the Finder. This makes it possible to import whole hierarchies of mail. The mailboxes are stored in the folder:
/Users/<username>/Library/Mail
(See How can I open the Library folder?.) Inside the V10 (or other V# folder for previous versions of macOS) is a folder for each account (named with a UUID or your e-mail address, and inside the account folder a folder for each mailbox.
When importing .mbox folders from Mail, you generally do not want to import via the To Import folder or the Files folder, as this would prevent EagleFiler from converting the mail to the standard mbox format. It is better to drag and drop to the Dock icon or into EagleFiler’s window, or to use the capture key from the Finder.
On macOS 10.14 and later, the first time you import from Mail’s folder you will need to grant EagleFiler Full Disk Access.