It's not that simple.
There are some tools to convert Maildir to mbox format.
Here's what they do...
They add a line at the top of each email that starts with the word "From" followed by a space, the email address of the sender, and the date the email was accepted at the server.
and...
If there are any lines inside the email that start with the word "From" followed by a space, they "escape" that line with a ">" symbol.
Why?
Because the way mbox works, it uses the word "From" followed by a space, as the way it knows where a new email begins.
The mailboxes themselves are stored in:
/var/spool/virtual/example.com/mailname
and the mailname file is owned by the domain owner and the mail group.
The filemode is 660.
Jeff