Please use "Field generator function" (advanced field settings) - you should code some method in custom module, install this module and set "Field generator function" to use that custom module method. You can find examples of such methods by searching for "QFfield_" in php files. The simplest one would be:
public static function QFfield_XXX(&$form, $field, $label, $mode, $default, $desc, $rb_obj) {
if($mode=='edit' || $mode=='add')
$form->addElement('hidden',$field,$default);
}