Can I show ads from external Google-certified ad networks on my pages and if so, where in my account to start setting it up? What do I click? Also: Is there anything I need to do prior to this? Like setting up ad space on my pages?
Yes, you can. AdSense Setup > Ad Review Center > CLICK "Change" under "Allow and Block Google Certified Ad Networks". Here you can allow or block / all or any specific network. Click the "Submit" button and you are done. None. If you are already using the adsense code generated, you just have to continue using them.