How do I migrate to a existing taxonomy reference field?
I've rewritten my yml to use:

field_special:
  plugin: iterator
  source: field_special
  process:
    target_id: tid


And written a preparerow function in my class to output an array as source property:

  /**
   * {@inheritdoc}
   */
  public function prepareRow(Row $row) {
    $nid = $row-&gt;getSourceProperty('nid');

    $result = $this-&gt;getDatabase()-&gt;query(
      'SELECT 
        GROUP_CONCAT(field_special_tid) as tids
      FROM
        {field_data_field_special} fs
      WHERE
        fs.entity_id = :nid
      ',
      array(':nid' =&gt; $nid)
    );
    foreach ($result as $record) {
      if (!is_null($record-&gt;tids)) {
        $row-&gt;setSourceProperty('special', explode(',', $record-&gt;tids));
      }
    }
    return parent::prepareRow($row);
  }


This workaround will do for now, but if you have a better answer please post so I can accept your answer.