Magento – Override Magento module layout “before/after”

When rendering the layout in Magento a few steps happen:

1. Handles are defined (default, catalog_product_view, etc.)
2. All XML files are loaded and merged to a single huge XML file, that file is stored somewhere in the cache.
3. The contents of all handles are loaded and merged into one page-xml file.
4. The order where the XML files are loaded are alphabetically and the local.xml is loaded last (there are also database layout updates which are loaded even later. (source https://github.com/OpenMage/magento-mirror/blob/magento-1.9/app/code/core/Mage/Core/Model/Layout/Update.php#L433-L438)
5. Finally the XML is parsed and the blocks is initiated.

So why isn't your example working? This is because the module XML file is loaded before the local.xml file. What happens is that the system first tries to add a block to a reference (to a block) which doesn't exist yet.

To solve te issue: You can use an update, updates get placed before the handles they are called in.

<default>
    <update handle="awesome_new_container_handle"/>
</default>

<awesome_new_container_handle>
    <reference name="not.important">
        <block type="core/text_list" name="extra.left.menu" as="extra" />
    </reference>
</awesome_new_container_handle>

And in your other module file add the block like your did.

<default>
    <reference name="extra.left.menu">
        <block type="myextension/some_block" template="some/template.phtml" as="some_name" name="some_name" />
    </reference>
</default>

Before header there is another block called after_body_start that directly comes after body-tag.

<reference name="after_body_start">
    <block type="company_name/module" name="page_header" before="-" />
</reference>