Hay que añadir datos en dos tablas wp_users y wp_usermeta.
INSERT INTO `databasename`.`wp_users` (`ID`, `user_login`, `user_pass`, `user_nicename`, `user_email`, `user_url`, `user_registered`, `user_activation_key`, `user_status`, `display_name`) VALUES ('4', 'demo', MD5('demo'), 'Syed Balkhi', 'test@yourdomain.com', 'http://www.wpbeginner.com/', '2011-06-07 00:00:00', '', '0', 'Syed Balkhi');
INSERT INTO `databasename`.`wp_usermeta` (`umeta_id`, `user_id`, `meta_key`, `meta_value`) VALUES (NULL, '4', 'wp_capabilities', 'a:1:{s:13:"administrator";s:1:"1";}');
INSERT INTO `databasename`.`wp_usermeta` (`umeta_id`, `user_id`, `meta_key`, `meta_value`) VALUES (NULL, '4', 'wp_user_level', '10');
Más info en: http://www.wpbeginner.com/wp-tutorials/how-to-add-an-admin-user-to-the-wordpress-database-via-mysql/