You can convert a given !OpenXmlElement to a XElement using the following code:
{{{#!highlight c#
OpenXmlElement el = ...; // Code to get the xml element from your office doc.

// Then use XElement.Parse and the OuterXml property.
XElement xel = XElement.Parse(el.OuterXml);
}}}

To convert an XElement to an !OpenXmlElement try the following code:
{{{#!highlight c#
XElement xe = ...;
using(StreamWriter sw = new StreamWriter(new MemoryStream()))
{
  sw.Write(xe.ToString());
  sw.Flush();
  sw.BaseStream.Seek(0, SeekOrigin.Begin);

  OpenXmlReader re = OpenXmlReader.Create(sw.BaseStream);

  re.Read();
  OpenXmlElement oxe = re.LoadCurrentElement();
  re.Close();
}

}}}
Hope, this helps.