How to include an existing XML file in a created DOMDocument

I create a new XML File with DOMDocument in PHP. That works fine. In between the document I want to add an already created XML File. How can I do this?

Existing XML Document:

    <?xml version="1.0" encoding="UTF-8"?>
    <books>
  <book>
  <magazine id="0" time="2014-02-18T14:42:06.0Z" type="0">New magazine</magazine>
  </book>
</books>

With the DOM functions of PHP I create another XML file. This works fine. Now I load the file above and create a fragement with:

$fragement = $document->createDocumentFragment();
$fragement->appendXML($this->xmlparser);

But now I dont know how to add the fragment to my document. I tried:

$document->documentElement->appendChild($fragement);

This has no effect on my XML Document.