test.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

class Foo
  def self.show
    'foo'
  end
end

module Bar
  def self.included(base)
    base.extend ClassMethods
    base.instance_eval do
      alias :show_without_bar :show
      alias :show :show_with_bar
    end
  end

  module ClassMethods
    def show_with_bar
      "#{show_without_bar} bar"
    end
  end
end

Foo.send :include, Bar
puts Foo.show #=> foo bar