bla.rb

require 'active_support/concern'

module MyCustomFoo
  extend ActiveSupport::Concern
  
  def foo_with_custom
    puts 'custom'
    foo_without_custom
  end
  
  included do
    alias_method_chain :foo, :custom
  end
end

class Foo
  def foo
    puts 'foo'
  end
end

Foo.class_eval do
  include MyCustomFoo
end

Foo.new.foo