This was adapted from an open issue on Github that shows how to perform an XDT config transform. The issue is still open, but I didn't there would need to be an extra dependency pulled in unless you really needed it and that is why it is still open.
#addin Cake.XdtTransform
var target = Argument("target", "Default");
Task("TransformConfig")
.Does(() =>
{
var sourceFile = File("web.config");
var transformFile = File("web.release.config");
var targetFile = File("web.target.config");
XdtTransformConfig(sourceFile, transformFile, targetFile);
});
RunTarget(target);