app.js

function koaSession() {
  return function(socket, next) {
    var req = socket.request;
    var res = new http.ServerResponse(req);
    var ctx = app.createContext(req, res);
    var gen = function *() { next(); };
    var sess = session();

    this.ctx = ctx;
    sess.call(ctx, gen()).next();
  };
}

io.use(koaSession());
io.use(function(socket, next) {
  console.log(this.ctx.session);
  next();
});