How to apply predefined app-service onto existing pools/VIPs?

I don't believe you can apply an iApp to existing pools/VIPs. An iApp is a template that builds your pools/VIPs/profiles/etc for you an puts it all in a container perse. https://devcentral.f5.com/wiki/iApp.HomePage.ashx

iApps cannot be introduced to any existing node or pool or vs, basically it is recommended to use iApps for fresh deployment of your app-services, so if you have an existing element you should delete those elements in advance, prior to iApps depoloyment