title: Add a custom content block within an existing content block of Magento Core
body: <p>I'm trying to add a custom order attribute, that the user can define on the checkout page just after the list of the payment methods and before the button continue.</p>
  
  <p>I want to do things as clean as possible, so I created my own module.</p>
  
  <p>Im struggling with  something I thought it would be simple but I don't succeed to find a solution.</p>
  
  <p>At the moment I only need to be able to display the template file in the following block .</p>
  
  <pre><code>Mage_Checkout_Block_Onepage_Payment
  </code></pre>
  
  <p>I created my template file which is only displaying a stupid text for the test
  app/design/defaut/defaut/template/mymodule/custom.phtml
  I know that there is no problem with this file while I used it on the index action of my module</p>
  
  <p>I think the main issue is that I must not do properly the xml layout declaration.</p>
  
  <p>Maybe you can help me </p>
  
  <pre><code>    &lt;?xml version="1.0"?&gt;
      &lt;layout version="0.1.0"&gt;
  &lt;!-- this part is working well --&gt;
          &lt;mymodule_index_index&gt;
              &lt;reference name="content"&gt;
                  &lt;block type="mymodule/mymodule" name="mymodule" template="mymodule/custom.phtml" /&gt;
              &lt;/reference&gt;
          &lt;/mymodule_index_index&gt;
  &lt;!-- this part is not working at all --&gt;
          &lt;checkout_onepage_index&gt;
              &lt;reference="checkout.onepage.payment" &gt;
                  &lt;block type="core/template" name="mymodule"  template="mymodule/custom.phtml" /&gt;
              &lt;/reference&gt;
          &lt;/checkout_onepage_index&gt;
      &lt;/layout&gt;
  </code></pre>
  
  <p>Of course my module is well declared as the index method of my module is working.
  is it my reference tag which is not good ? </p>
  
  <p>Thanks in advance, this layout organisation gives me headache,</p>
  
  <p>Best,</p>
  
  <p>Anselme</p>