There is a feature to do that go in to config/main.inc.php then find:
// display these folders separately in the mailbox list.
// these folders will automatically be created if they do not exist
$rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk', 'Trash');
// protect the default folders from renames, deletes, and subscription changes
$rcmail_config['protect_default_folders'] = TRUE;