Create XML Nodes based on XPath?

In the example you present the only thing being created is the attribute ...

XmlElement element = (XmlElement)doc.SelectSingleNode("/feed/entry/content");
if (element != null)
    element.SetAttribute("source", "");

If what you really want is to be able to create the hierarchy where it doesn't exist then you could your own simple xpath parser. I don't know about keeping the attribute in the xpath though. I'd rather cast the node as an element and tack on a .SetAttribute as I've done here:

static private XmlNode makeXPath(XmlDocument doc, string xpath)
{
    return makeXPath(doc, doc as XmlNode, xpath);
}

static private XmlNode makeXPath(XmlDocument doc, XmlNode parent, string xpath)
{
    // grab the next node name in the xpath; or return parent if empty
    string[] partsOfXPath = xpath.Trim('/').Split('/');
    string nextNodeInXPath = partsOfXPath.First();
    if (string.IsNullOrEmpty(nextNodeInXPath))
        return parent;

    // get or create the node from the name
    XmlNode node = parent.SelectSingleNode(nextNodeInXPath);
    if (node == null)
        node = parent.AppendChild(doc.CreateElement(nextNodeInXPath));

    // rejoin the remainder of the array as an xpath expression and recurse
    string rest = String.Join("/", partsOfXPath.Skip(1).ToArray());
    return makeXPath(doc, node, rest);
}

static void Main(string[] args)
{
    XmlDocument doc = new XmlDocument();
    doc.LoadXml("<feed />");

    makeXPath(doc, "/feed/entry/data");
    XmlElement contentElement = (XmlElement)makeXPath(doc, "/feed/entry/content");
    contentElement.SetAttribute("source", "");

    Console.WriteLine(doc.OuterXml);
}

Here's my quick hack that can also create attributes as long as you use a format like /configuration/appSettings/add[@key='name']/@value.

static XmlNode createXPath(XmlDocument doc, string xpath)
{
  XmlNode node=doc;
  foreach (string part in xpath.Substring(1).Split('/'))
  {
    XmlNodeList nodes=node.SelectNodes(part);
    if (nodes.Count>1) throw new ComponentException("Xpath '"+xpath+"' was not found multiple times!");
    else if (nodes.Count==1) { node=nodes[0]; continue; }

    if (part.StartsWith("@"))
    {
      var anode=doc.CreateAttribute(part.Substring(1));
      node.Attributes.Append(anode);
      node=anode;
    }
    else
    {
      string elName, attrib=null;
      if (part.Contains("["))
      {
        part.SplitOnce("[", out elName, out attrib);
        if (!attrib.EndsWith("]")) throw new ComponentException("Unsupported XPath (missing ]): "+part);
        attrib=attrib.Substring(0, attrib.Length-1);
      }
      else elName=part;

      XmlNode next=doc.CreateElement(elName);
      node.AppendChild(next);
      node=next;

      if (attrib!=null)
      {
        if (!attrib.StartsWith("@")) throw new ComponentException("Unsupported XPath attrib (missing @): "+part);
        string name, value;
        attrib.Substring(1).SplitOnce("='", out name, out value);
        if (string.IsNullOrEmpty(value) || !value.EndsWith("'")) throw new ComponentException("Unsupported XPath attrib: "+part);
        value=value.Substring(0, value.Length-1);
        var anode=doc.CreateAttribute(name);
        anode.Value=value;
        node.Attributes.Append(anode);
      }
    }
  }
  return node;
}

SplitOnce is an extension method:

public static void SplitOnce(this string value, string separator, out string part1, out string part2)
{
  if (value!=null)
  {
    int idx=value.IndexOf(separator);
    if (idx>=0)
    {
      part1=value.Substring(0, idx);
      part2=value.Substring(idx+separator.Length);
    }
    else
    {
      part1=value;
      part2=null;
    }
  }
  else
  {
    part1="";
    part2=null;
  }
}

Sample:

public static void Set(XmlDocument doc, string xpath, string value)
{
  if (doc==null) throw new ArgumentNullException("doc");
  if (string.IsNullOrEmpty(xpath)) throw new ArgumentNullException("xpath");

  XmlNodeList nodes=doc.SelectNodes(xpath);
  if (nodes.Count>1) throw new ComponentException("Xpath '"+xpath+"' was not found multiple times!");
  else if (nodes.Count==0) createXPath(doc, xpath).InnerText=value;
  else nodes[0].InnerText=value;
}

e.g.

Set(doc, "/configuration/appSettings/add[@key='Server']/@value", "foobar");