dovecot - Sep 2012 - thunderbird does not copy a tree-structure to dovecot imap

Hi,

running in problems with KMail I decided to switch to 
dovecot-imap & thunderbird on my kubuntu 12.04 64 bit
(so I can read my imap-folder-mails with KMail again if it gets more stable)

What I did:
 - I wrote a python script to convert my KMail mails folder to a 
   thunderbird mailfolder (directory tree containing mbox files 
   and *.sbd folders)

 - I created a fake pop mail account in thunderbird and imported the
   whole mail-structure (i just copied it)

 - I create a local imap account ( so I have a ~/Maildir dovecot 
   folder in my user dir)

 - In /etc/dovecot:

   1. /etc/dovecot/10-mail.conf (line 10) I set:
      mail_location = maildir:~/Maildir:LAYOUT=fs

   2. /etc/dovecot/20-imap.conf (line 57) I set:
      imap_client_workarounds = tb-extra-mailbox-sep

Now I want to copy the whole mailstructure from my imported
fake pop mailfolder to the dovecot imap folder.

What happens:

Lets say my source structure is

 mailfolder1
   + mail1
   + mail2
   + subMailFolder2
       + mail1-in-subMailFolder2
       + mail2-in-subMailFolder2
       + subSubMailFolder3
           + mail1-in-subMailFolder3
           + mail2-in-subMailFolder3
    
I thunderbird (version 15) I drag and drop my mailFolder1 
to the imap folder and I get:

 mailfolder1
   + mail1
   + mail2
   + subMailFolder2
       + subSubMailFolder3

as you See only the mails of the *TOP* folder are copied.
All subfolders are created but they contain NO MAILS.

Did somebody experience the same problem???

Otherwise I will write a python script to do that
(I dont hope to need to write a mailclient in python ;-) )

Any hint is greatly welcome.

  Anton