Thanks for the information.
We have to about 2,000 IMAP accounts to create and then transfer stored mails from an external hosting provider to these new IMAP accounts. The users use Evolution and Thunderbird on very low end diskless Gentoo systems. The sub accounts below /maildir/ should be automatically created when the user connect the first time with his Evolution or Thunderbird mail client and create the aforesaid directory structure, including /.INBOX.Junk.
It can be easily be done with IRedMail, but requires a dedicated mail server, hardware and administration.