code: public function addField($name, $value)
      {
          if ($this->currentRow < 0) {
              $this->appendRow();
          }
          $this->collection[$this->currentRow]->addField($name, $value);
      }
docs: Add a single string field to an existing row
  
  @param string $name - Field name
  @param string $value - Field value
  @return void
  @throws \ByJG\Serializer\Exception\InvalidArgumentException