[Dovecot] IMAP subfolders and MBOX format

Hello folks

I have a problem related to the mbox format I think

First we use MBOX format with Dovecot 1.14

A user's here need to create some subfolders in his Imap space.

Due to the use of MBOX format this is impossible from thunderbird as you cannot creat a directory in a file :-)

So to help him I created manually on the mail server some directories into his imap space,

Doing so we are able to see the folder and subfolders but it is not possible to move some other folders into the subfolder via the thunderdird client.

Anyone to help me a bit ?

Note : on another machine which use the Maildir format I can do whatever I want without problem.

Thanks a lot.