The simplest way to add this attribute is use XML.
This sample part of code displayed below, should be inserted in Your own module in config.xml file

<frontend>
<category>
<collection>
<attributes>
<custom_attribute_code/>
</attributes>
</collection>
</category>
</frontend>

After that You can use this attribute, in rendering method _renderCategoryMenuItemHtml defined in Mage_Catalog_Block_Navigation.
Like that:

Please remember to rewrite this class, or at least move it to app/code/local/.. directory.

  • mathias

    You’ve saved my day, master! 🙂