When you add them to the validfields array, you may then use the datamanager to 'set' them prior to saving them. So, in threaddata_start, you add them:
PHP Code:
$this->validfields['whatever'] = array(TYPE_UINT, REQ_NO);
And then in your plugin, you set them:
PHP Code:
$threadman->set('whatever', $vbulletin->GPC['whatever']);
You would do that prior to them being saved:
PHP Code:
$threadman->save();
Did you do as I suggested and look at other modifications to see how they do this?