It's already in place... BatchFB will auto-detect that you are on appengine and use async fetching. Look at RequestExecutor.java if you want to see how it works.
If you're not on GAE and you want async fetching, you just have to implement RequestExecutor with whatever flavor of async your container supports and call RequestExecutor.setInstance(). The default (non-appengine) executor just uses HttpURLConnection.
Jeff