You could do something like this:

HtmlDocument doc = new HtmlDocument();
doc.Load(MyTestHtm);
HtmlNode body = doc.DocumentNode.SelectSingleNode("//body");
if (body == null)
{
    HtmlNode html = doc.DocumentNode.SelectSingleNode("//html");
    // we presume html exists

    body = CloneAsParentNode(html.ChildNodes, "body");
}


static HtmlNode CloneAsParentNode(HtmlNodeCollection nodes, string name)
{
    List<HtmlNode> clones = new List<HtmlNode>(nodes);
    HtmlNode parent = nodes[0].ParentNode;

    // create a new parent with the given name
    HtmlNode newParent = nodes[0].OwnerDocument.CreateElement(name);

    // insert before the first node in the selection
    parent.InsertBefore(newParent, nodes[0]);

    // clone all sub nodes
    foreach (HtmlNode node in clones)
    {
        HtmlNode clone = node.CloneNode(true);
        newParent.AppendChild(clone);
    }

    // remove all sub nodes
    foreach (HtmlNode node in clones)
    {
        parent.RemoveChild(node);
    }
    return newParent;
}