<Table>
  <TableColumn Width="331"/>
  <TableColumn Width="48"/>
  <TableColumn Width="48"/>
  <TableColumn Width="48"/>
  <TableColumn Width="48"/>
  <TableColumn Width="44"/>
  <TableColumn Width="52"/>
  ...
</Table>

<Table>
  <Table.Columns>
    <TableColumn Width="331"/>
    <TableColumn Width="48"/>
    <TableColumn Width="48"/>
    <TableColumn Width="48"/>
    <TableColumn Width="48"/>
    <TableColumn Width="44"/>
    <TableColumn Width="52"/>
  </Table.Columns>
  ...
</Table>

XDocument doc1 = XDocument.Parse(str);
XNamespace ns = doc1.Root.Name.Namespace;
IEnumerable<XElement> nodes = doc1.Descendants(ns+"Table");
foreach (XElement noded in nodes)
                {
                    XElement node3 = new XElement(ns + "Table.Columns");
                    IEnumerable<XElement> nodes2 = from el in noded.Elements(ns+"TableColumn")    select el;
                    if (nodes2.Count() > 0)
                    {
                        node3.AddFirst(nodes2);
                        noded.AddFirst(node3);
                        nodes2.Remove();
                    }
                }