What did i do?
I have download all files of my current hosting (/home/[user]/) to /tmp/[user] of the new server
and ran this:
./mb2md -s /tmp/[user]/ -R -d /var/vmail/[domain]/[user]/Maildir/
When i check my roundcube on my new server, all my mails are imported.
1 disadvantage: all my mails are marked unread
Hopefully this will help anyone.