Magento: Override only a part of a layout xml-File - magento

Question

I only want to change a part of the file checkout.xml (this file is in the core layout-folder). I create my own extension and link to my own layout-xml-File in my config.xml.
<layout>
<updates>
<checkout>
<file>mymodule_checkout.xml</file>
</checkout>
</updates>
</layout>
There are no problems, when i copy the whole file checkout.xml and save it as mymodule_checkout.xml. But I only want to override a little part of this big file between
<reference name="content">
and
</reference>
In this case, when i only write the parts that i needed, magento only read my file with this parts. So magento don't get the remaining parts of the original checkout.xml.
Is it possible to only change a part of a core layout xml-File without copying the whole file or change the core.
The Code of mymodule.xml looks like this:
<layout version="0.1.0">
<checkout_onepage_index translate="label">
<reference name="content">
<block type="checkout/onepage" name="checkout.onepage" template="checkout/onepage.phtml">
<block type="checkout/onepage_login" name="checkout.onepage.login" as="login" template="checkout/onepage/login.phtml">
<block type="page/html_wrapper" name="checkout.onepage.login.before" as="login_before" translate="label">
<label>Login/Registration Before</label>
<action method="setMayBeInvisible">
<value>1</value>
</action>
</block>
<block type="checkout/onepage_billing" name="checkout.onepage.billing" as="billing" template="checkout/onepage/billing.phtml"/>
<block type="checkout/onepage_shipping" name="checkout.onepage.shipping" as="shipping" template="checkout/onepage/shipping.phtml"/>
</block>
<block type="checkout/onepage_shipping_method" name="checkout.onepage.shipping_method" as="shipping_method" template="checkout/onepage/shipping_method.phtml">
<block type="checkout/onepage_shipping_method_available" name="checkout.onepage.shipping_method.available" as="available" template="checkout/onepage/shipping_method/available.phtml"/>
<block type="checkout/onepage_shipping_method_additional" name="checkout.onepage.shipping_method.additional" as="additional" template="checkout/onepage/shipping_method/additional.phtml"/>
</block>
<block type="checkout/onepage_payment" name="checkout.onepage.payment" as="payment" template="checkout/onepage/payment.phtml">
<block type="checkout/onepage_payment_methods" name="checkout.payment.methods" as="methods" template="checkout/onepage/payment/methods.phtml">
<action method="setMethodFormTemplate">
<method>purchaseorder</method>
<template>payment/form/purchaseorder.phtml</template>
</action>
</block>
</block>
<block type="checkout/onepage_review" name="checkout.onepage.review" as="review" template="checkout/onepage/review.phtml"/>
</block>
</reference>
</checkout_onepage_index>
</layout>
Thats the part I need. In this Case, magento take only this code not the remaining code of the core xml-file.

Answer 1

Absolutely, yes. Magento combines all your layout files into one, including files from fallback themes. Whatever is still relevant after it's processed all your files is what is used to produce your layout and as long as your xml is formatted correctly you can override any section from any .xml file - i.e. just 'cause it's called page.xml doesn't mean you can't override it in foobarbaz.xml.
It's more than likely your problem lies with your XML, can you post the contents of mymodule_checkout.xml so I can figure out what you're trying to do and hopefully provide you with a working replacement?