Overriding a gem's controller method in Rails 5

I'm using a (private) gem which defines a Rails 5.2 controller. I want to override the private params permitting method in the mentioned controller to add some different params.

I tried to reference the Rails Engine Guide but it doesn't actually show how to override a controller method (only a model, and the same approach doesn't seem to work for controllers.)

Update: Here's the decorator pattern I tried based on the aforementioned Rails guide.

1. Load the decorators (Is the /lib folder even loaded by Rails anymore?):

# MyApp/lib/private_gem/engine.rb
module PrivateGem
  class Engine < ::Rails::Engine
    isolate_namespace PrivateGem

    config.to_prepare do
      Dir.glob(Rails.root + "app/decorators/**/*_decorator*.rb").each do |c|
        require_dependency(c)
      end
    end
  end
end

2. The Decorator:

# MyApp/app/decorators/controllers/private_gem/users_controller_decorator.rb

PrivateGem::UsersController.class_eval do
  private

  # Only allow a trusted parameter "white list" through.
  def user_params
    params.require(:user).permit(:existing_param, :new_param)
  end
end

I have a working solution that I don't like. First, I load the existing class and then redefine the private method.

# MyApp/controllers/private_gem/users_controller.rb

load PrivateGem::Engine.root.join('app/controllers/private_gem/users_controller.rb')
PrivateGem::UsersController.class_eval do
  private

  # Only allow a trusted parameter "white list" through.
  def user_params
    params.require(:user).permit(:existing_param, :new_param)
  end
end

Is this really the best way to do this? Maybe it belongs in a different place? Should I somehow use ActiveSupport::Concern? It doesn't seem like idiomatic Rails to me but I have a lot to learn about how Rails is initialized and how Engines and Concerns work.

Thanks!