[Dovecot] merge mdboxes

Hi there, after migration we have a very big problem... there are two mailboxes in /var/mail/username/mailboxes/ /var/mail/username/mailboxes/inbox /var/mail/username/mailboxes/inbox2 i DO have to merge these boxes, but i found no tools for this step. is
there any way with doveadm oder dsync to move alle messages from mdbox
inbox2 into mdbox inbox? i hope you all can tell me a way to do this - its a very very big
problem at the moment. By the way - logging in via imap with thunderbird und move them on
that way is not my favorite solution cause i have to do this for
hundreds of users... i hope you can help me! thanks a lot! Tobias Daucher