redefine_spree_user_params.rb

## in main app

Spree::UserRegistrationsController.class_eval do 
  include SpreeMailchimpGibbon::SpreeUserRegistrationsControllerConcern
end

## in spree_mailchimp_gibbon extension

module SpreeMailchimpGibbon
  module SpreeUserRegistrationsControllerConcern
    extend ActiveSupport::Concern
    
    private 
      def spree_user_params
        params.require(:spree_user).permit(:email, :password, :password_confirmation, :is_mail_list_subscriber)
      end 
  end
end