I would be very very careful with modifying the behaviour of objects like this. It is typically better to provide a wrapper for localStorage (or use store.js or another library) to provide the features you want.
In the case that you do want to add a method or property to localStorage, you can do so by adding it to it's constructor's prototype:
typeof(localStorage.prototype); // "undefined"
localStorage.constructor // function Storage() { [native code] }
Storage.prototype.foo = function () { return 'foo'; }
// setting a method on the constructor allows each localStorage instance
// to inherit and use it
localstorage.foo() // 'foo'