How can this be done without calling a private method?

doesnt_work_1.rb

# app/models/registration.rb

class Registration
  attr_accessor :current_step
end

# Registration then ONLY has :current_step in it and the other methods from the engine are blasted

extensions.rb

# config/initializers/extensions.rb
# ---
# Registration is defined as a model in an engine.
# On the local app I want to add :current_step as an attr so I can use it in the interface.
# Without adding it to the engine model how can I extend the class without resorting to calling include like this.
# Ideally id love to put this into app/models/registration.rb but "X file expects to define class Y" is a bitch.

module Extension
  module Registration
    attr_accessor :current_step
  end
end

Registration.send(:include, Extension::Registration)