Add custom attributes to collections with XML

attributesXML

/** Add custom attribute to category collection on frontend */
<frontend>
    <category>
        <collection>
            <attributes>
                <some_category_attribute/>
            </attributes>
        </collection>
    </category> 
</frontend>

/** Add custom attribute to product collection on frontend */
<frontend>
    <product>
        <collection>
            <attributes>
                <some_product_attribute/>
            </attributes>
        </collection>
    </product> 
</frontend>

/** Add attributes to any config node that permits it e.g sales_quote_item */
<global>
    <sales>
        <quote>
            <item>
                <product_attributes>
                    <some_product_attribute />
                </product_attributes>
            </item>
        </quote>
    </sales>
</global>