It might be too late but I leave my solution. I added it as a ElementExtension then it works for me. My environment is .NET 4.5.
XNamespace nsDefault = "http://www.w3.org/2005/Atom";var content = new XElement(nsDefault +"content");content.Add(new XCData("<b>Item Content</b>"));item.ElementExtensions.Add(new SyndicationElementExtension(content));