appmodelsuser.rb

class User
  include Mongoid::Document
  
  field :email, :type => String
end

configinitializerstest.rb

require 'lib/test/test'
User.send :include, Test::User

Console

ree-1.8.7-2011.03 :005 > Card.new
 => #<Card _id: 4ec4f9ff31ef62a8ee000003>

ree-1.8.7-2011.03 :001 > u = User.new
 => #<User _id: 4ec4f9ea31ef62a8ee000001, email: nil>

ree-1.8.7-2011.03 :003 > u.cards
NoMethodError: undefined method `cards' for #<User:0x12d3170d0>

libtestappmodelscard.rb

class Card
  include Mongoid::Document

  belongs_to :user
end

libtestextsuser.rb

module Test
  module User
    extend ActiveSupport::Concern

    included do |base|
      base.send(:has_many, :cards)
    end

    module ClassMethods
    end

    module InstanceMethods
  end
end

libtesttest.rb

require 'lib/test/app/models/card'
require 'lib/test/exts/user'