I am using System.ServiceModel.Syndication.SyndicationFeed to create an rss feed from which I get this:
To specify a custom name for the atom extensions you need to disable SerializeExtensionsAsAtom on the feed formatter:
var formatter = feed.GetRss20Formatter();
formatter.SerializeExtensionsAsAtom = false;
Then you need to add the namespace
XNamespace atom = "http://www.w3.org/2005/Atom";
feed.AttributeExtensions.Add(new XmlQualifiedName("atom", XNamespace.Xmlns.NamespaceName), atom.NamespaceName);
And now you can start using the extensions
feed.ElementExtensions.Add(new XElement(atom + "link", new XAttribute("href", feedLink), new XAttribute("rel", "self"), new XAttribute("type", "application/rss+xml")));
Finally write the feed to the response stream:
formatter.WriteTo(new XmlTextWriter(Response.Output));