Hi,
First of all this has nothing to do with roundcube, roundcube is an email client (installed somewhere or on the emailserver to asscess either dovecot cyrus or other). What will be important to know is which email-server are you using (is it dovecot or cyrus or other ?) and is it hosted or on premises (if hosted you will most likely have to rely on others to assist).
Mass migration is easy with dovecot doveadm tool, for cyrus there are scripts.
You could also use a impa migration tool (if you google for thtat you get plenty of results).
Regards,