class A
  def initialize(add_module)
    self.class.send(:include, MM::NN) if add_module
  end
end

module MM::NN
  def display
    puts "module method called"
  end
end

A.new(true).display  #module method called
A.new(false).display  #NoMethodError

A.new(true).display  #module method called
A.new(false).display  #NoMethodError

A.new(false).display  #NoMethodError
A.new(true).display  #module method called

module TestModule
  def test_method; "ok"; end
end

class A
    def initialize(add_module)
        singleton_class.include(TestModule) if add_module
    end
end

A.new(true).test_method  # module method called
A.new(false).test_method # NoMethodError

class A
  def initialize(add_module)
    extend(MM::NN) if add_module
  end
end

class A
  def initialize(add_module)
    if add_module
      (class <<self; include MM::NN; end)
    end
  end
end

# This class is here because I'm assuming you can't make `MM::NN` a class in its own right. However it probably needs additional data or methods to actually work with the MM::NN methods
class NN_implementation
  include MM::NN
end

class A
  def initialize(use_module)
    # if this class needs access to data or methods from A, you could add `self` as an input arg to the constructor of `NN_implementation`
    @nn = NN_implmenetation.new if use_module
  end

  def has_nn?
    !@nn.blank?
  end
end