Roundcube is an email client, not an email server or system. Did you install and configure an email (IMAP) server as well? You will need the IMAP server for Roundcube to work at all, and the accounts and messages you wish to move need to be migrated to your IMAP server, not into Roundcube. How that can be done depends on the IMAP server choice that you have set up, as well as what server software register.it uses.