How do I add validation in the node processor before creating a node?
You can add this functionality in a custom module. But you'll have to add the logic that you have in your node's validation here.

/*
 * Implements hook_feeds_prevalidate()
 */
function mymodule_feeds_prevalidate($source, &amp;$entity, &amp;$item, $entity_id) {
  // the $item contains your csv files column values
  if ($entity-&gt;feeds_item-&gt;id == 'import_name') { // check the feed import ID
    if ($item['some column'] != 'something') { // validate column value
      $entity-&gt;feeds_item-&gt;skip = TRUE; // skips this row from importing
    }
  }
}


Important: I think you'll need 7.x-2.x-dev in order for this hook to work, I'm using it. This is a new hook and I think it has not been added to 7.x-2.0-beta4 yet.