var newNode = root.ImportNode(childItem.FirstChild, true);
root["tables"]["table"].AppendChild(newNode);

var newNode = root.ImportNode(childItem.FirstChild, true);

var document1 = new XmlDocument();
document1.LoadXml("<tables><table></table></tables>");

var childDocument = new XmlDocument();
childDocument.LoadXml(string.Format(@"<item>
    <column columnName=""Article_text"">{0}</column>
    <column columnName=""Article_name"">{1}</column>
    </item>", atext, aname));

 document1.AppendChild(childDocument.ChildNode);