<?xml version="1.0" ?>
<config>
    <modules>
        <Bromo_Orderstatus>
            <active>true</active>
            <codePool>local</codePool>
        </Bromo_Orderstatus>
    </modules>
</config>

<config>
    <modules>
        <Bromo_Orderstatus>
            <version>1.0.0</version>
        </Bromo_Orderstatus>
    </modules>
    <global>
        <blocks>
             <adminhtml>
                <rewrite>
                    <sales_order_view>Bromo_Orderstatus_Block_Adminhtml_Sales_Order_View</sales_order_view>
                </rewrite>
            </adminhtml>
        </blocks>
    </global>
    <admin>
        <routers>
            <adminhtml>
                <use>admin</use>
                <args>
                    <modules>
                        <orderstatus before="Mage_Adminhtml_Sales">Bromo_Orderstatus_Adminhtml</orderstatus>
                    </modules>
                </args>
            </adminhtml>
        </routers>
    </admin>
</config>

<?php
class Bromo_Orderstatus_Block_Adminhtml_Sales_Order_View extends Mage_Adminhtml_Block_Sales_Order_View {
    public function  __construct() {
        $message = Mage::helper('sales')->__('Weet je zeker dat je de klant een klaar voor afhalen e-mail wil versturen?');
        $this->_addButton('afhalenmail', array(
            'label'     => Mage::helper('Sales')->__('Stuur afhaal E-mail (nog niet gebruiken!)'),
            'onclick'   => "confirmSetLocation('{$message}', '{$this->getUrl('*/*/afhalenmail')}')"
            ));
        parent::__construct();
    }
}

<?php
require_once 'Mage/Adminhtml/controllers/Sales/OrderController.php';

class Bromo_Orderstatus_Adminhtml_Sales_OrderController extends Mage_Adminhtml_Sales_OrderController {

    public function afhalenmailAction() {

         echo 'Test action';

    }

}