AppEngine Cloud Endpoints and custom Users service

Question

Is it possible to use a custom Users service (a service that implements UsersService interface) with Google AppEngine endpoints?

Let's take the example from Google AppEngine website

@ApiMethod(name = "greetings.authed", path = "greeting/authed")
public HelloGreeting authedGreeting(User user) {
    HelloGreeting response = new HelloGreeting("hello " + user.getEmail());
    return response;
}

When a request is made to the application and this endpoint is hit it somehow contacts the Users service and gets the information about authenticated user (because the OAuth token is available in the request).

I'd like to somehow use my own service which would implement UsersService interface and provide me with a user object.