Scaffold invoking Custom Generator

Hi Guys,

I created a custom generator and I would like hook it to the scaffold
generator, so whenever the scaffold generator is called, my generator is
also called.

To achieve that i did this:

config.generators do |g|
g.helper :my_custom

It works, but I have to concerns:

First, I had to replace one generator (helper). Is possible to add a new
generator to the scaffold call, instead of replace one?

Second, I know that some gems like RSpec and Haml are able to replace
generators (test_framework and templates, respectively) without change
config/application.rb, how is that possible?