Append XML string block to existing XmlDocument

I suggest using XmlDocument.CreateDocumentFragment if you have the data in free form strings. You’ll still have to use AppendChild to add the fragment to a node, but you have the freedom of building the XML in your StringBuilder.

XmlDocument xdoc = new XmlDocument();
xdoc.LoadXml(@"<MyXml><Employee></Employee></MyXml>");

XmlDocumentFragment xfrag = xdoc.CreateDocumentFragment();
xfrag.InnerXml = @"<Demographic><Age/><DOB/></Demographic>";

xdoc.DocumentElement.FirstChild.AppendChild(xfrag);