magento: insert content in to predefined block using Custom Layout Update

I have the Ultimo theme on my magento store. There is a predefined static block called 'block_product_secondary_bottom' that when enabled will show up on all products. I am trying to show content in this block's space for select products, and from another defined static block named 'suggested_addons'.

I used the following code in Custom Layout Update and it doesn't work. Any suggestions?

<reference name="block_product_secondary_bottom">
    <block type="cms/block" name="suggested_addons">
        <action method="setBlockId"><block_id>suggested_addons</block_id></action>
    </block>    
</reference>