As per my knowledge -- no way.
Read this extended answer to my question, to get a brief discussion on most popular mobile ads networks. As you can see from this text, MobFox, is the only (known to me) mobile ads provider, that supports displaying ads via JavaScript.
If you want to display AdMob ads in your iOS app, you have to compile your app directly on local version of PhoneGap (not through PhoneGap Build), which will give you access to external container for your HTML5 content. You can then modify Java code of that container to add AdMob adds to your app.
There were many articles written about this (like the one at PhoneGap Wiki). You will also find many answers here in StackOverflow, posts in many blogs and information in many sources in the Interent.