You will still need to have (create) the mailbox or maildir for the mail users. Creating the SQL entry in roundcube won't do this part.
I, personally, would recommend this tool: http://sourceforge.net/projects/postfixadmin/
It is a web based user management tool for postfix that will do a lot of the user database maintenance for you. If you use it, be sure that you take proper steps to secure it, like making it accessible only on localhost (not accessibly from the Internet) and require strong credentials or a browser certificate.