So, it doesn't appear there's a clean way to add functionality to modules that are already loaded. This happens in 1.9.3 and 2.0.0.

adding_methods_to_enumerable.rb

module MyModule
  def my_module_method
    puts "BOOYAH!!"
  end
end

Enumerable.send :include, MyModule
Enumerable.instance_methods.include? :my_module_method
#=> true

Hash.new.my_module_method
#=> NoMethodError: undefined method `my_module_method' for {}:Hash

module Enumerable
  def my_direct_method
    puts "WTF?!"
  end
end

Enumerable.instance_methods.include? :my_direct_method
#=> true

Hash.new.my_direct_method
#=> WTF?!

module Enumerable
  def my_module_method
    self.extend MyModule
    my_module_method
  end
end

Hash.new.my_module_method
#-> BOOYAH!!