<config>
  ...

    <admin>
        <routers>
            <adminhtml>
                <args>
                    <modules>
                        <store before="Mage_Adminhtml">Mycompany_Store_Adminhtml_Store</store>
                    </modules>
                </args>
            </adminhtml>
        </routers>
</admin>
 </global>
....
 </config>

<sections>
    <general>
        <groups>
            <store_information>
                <fields>
                    <commission>
                        <label>Commission</label>
                        <frontend_type>text</frontend_type>
                        <sort_order>10</sort_order>
                        <show_in_default>1</show_in_default>
                        <show_in_website>1</show_in_website>
                        <show_in_store>1</show_in_store>
                    </commission>
                </fields>
            </store_information>
        </groups>
    </general>
</sections>

<admin>
        <routers>
            <adminhtml>
                <args>
                    <modules>
                        <Mycompany_Store before="Mage_Adminhtml">Mycompany_Store_Adminhtml</Mycompany_Store>
                    </modules>
                </args>
            </adminhtml>
        </routers>
</admin>

<?php
require_once 'Mage/Adminhtml/controllers/System/StoreController.php';
class Mycompany_Store_Adminhtml_System_StoreController extends Mage_Adminhtml_System_StoreController{
    //your methods here
}

<global>

        <rewrite>
            <mycompany_store_adminhtml_system_storecontroller>
                <from><![CDATA[#^/admin/system_store/#]]></from> <!-- Mage_Adminhtml_System_StoreController  -->
                <to>/store/adminhtml_system_store/</to> <!-- Mycompany_Store_Adminhtml_System_StoreController  -->
            </mycompany_store_adminhtml_system_storecontroller>
        </rewrite>
</global>