Answered Adding extra fields with join not showing.

class mpcOrderGetListProcessor extends mpcObjectGetListProcessor {

  public $classKey = 'mpcOrder';
  public $languageTopics = array();
  public $defaultSortField = 'docnumber';
  public $defaultSortDirection = 'ASC';
  public $objectType = 'mpcOrder';
  public $cacheFolder = 'mpcOrder';

  /**
   * @param xPDOQuery $c
   * @return \xPDOQuery
   */
  public function prepareQueryBeforeCount(xPDOQuery $c) {

    $where = $this->getProperty('where');

    $c->leftJoin('mpcOrderLines', 'mpcOrderLines', array('`mpcOrder`.`docnum` =`mpcOrderLines`.`docnum`'));
    $c->where(array('mpcOrderLines.deliverystatus' => 1));
    if (!empty($where)) {
      $where = json_decode($where, true);
      if (is_array($where)) {
        foreach ($where as $wkey => $wsearch) {
          $c->where(array($wkey => $wsearch));
        }
      }
    }

    return $c;
  }

  public function prepareRow(xPDOObject $object){
	  $ta = $object->toArray('',false,true);
	  return $ta;
  }

  public function prepareQueryAfterCount(xPDOQuery $c) {

    $c->select($this->modx->getSelectColumns('mpcOrder', 'mpcOrder'));
    $c->select($this->modx->getSelectColumns('mpcOrderLines', 'mpcOrderLines'));
    return $c;
  }

}

return 'mpcOrderGetListProcessor';

$c->select($this->modx->getSelectColumns('mpcOrderLines', 'mpcOrderLines','Lines_'));