admin \ packages.rb

ActiveAdmin::Register Package do 
  # A custom action we want treated like the :show or :edit actions.
  member_action :overview, :method => :get do
    @package = Package.find params[:id]
  end

  # Add the actions for the :overview action.
  template_actions_for :overview, :show
end

monkeypatches \ active_admin.rb

class ActiveAdmin::DSL
  def template_actions_for action, template_action
    config.template_actions_for action, template_action
  end
end

module ActiveAdmin
  class Resource
    module ActionItems
      def template_actions_for action, template_action
        action_items_for(template_action).each do |action_item|
          add_action_item :only => [action], &action_item.block
        end
      end
    end
  end
end