Well, mojito does a poor job exposing some of the underlaying APIs, and express is one of them, and we are working to fix that part. However, you do have access to the express obj in your server.js by doing something like this:
var app = Mojito.createServer();
// app._app is an express server :)
@Sai Prasad, if you use mojito start you're not using server.js, this is confusing, and we are working on splitting the cli out of mojito, so mojito start will become a sugar on top of server.js. To start with server.js just use node server.js.