If someone needs to add a xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" to a xml element using Linq to XML all you have to do is something like this
XNamespace xsi = "http://www.w3.org/2001/XMLSchema-instance";
XDocument doc = new XDocument(
new XElement("Root",
new XAttribute(XNamespace.Xmlns + "xsi", xsi)));
maybe it's not very intuitive but works :)