insert XmlDocument into a XmlDocument node

If I recall correctly that it's basically the same thing in every DOM Implementation around (.net, javascript, php etc. this should work.

XmlNode requestNode =  bigDoc.FirstChild;
requestNode.AppendChild(
    requestNode.OwnerDocument.ImportNode(
        anotherXMLDocument.DocumentElement, true));

The true (2nd argument to importNode) should mean import deep.