Installed courier-imap-126.96.36.19921124 for testing "fake e-mail accounts".
Installation instructions: http://www.inter7.com/courierimap/INSTALL.html#userdb
Now I can simply put RFC822 text files into maildir/new, and they appear in the IMAP inbox (or in the POP3 inbox)!
Use this for push profiles (SDI), e-mail archiving (the user can simply drag&drop mails into this folder using his mail client), ...
User accounts are defined in /etc/userdb, need programs in /usr/lib/courier-imap/sbin (userdb, userdbpw, makeuserdb) for setting them up.