You'd have to tinker with the AGKHelper.java file a bit to accomplish this. If you're using T1 then you'd need to find a command that you aren't using to transmit data to AGKHelper.java. For example, you could use SetAdmobDetails to actually send data to what is currently coded for admob, but you'd need to alter it in the java file to actually use the Google play Game Services.
In T2 it would be "easier" since you could write your own commands that would set the data in AGKHelper.java (or whatever activity you wanted to use).
With all that said, you might want to look into using the web API. You'll probably still need to put some stuff in the java files to set up your app (like what is done for IAP). But you can then use the web API to actually send data to Google Play Game Services. This is a lot like how push is done. Should be pretty doable and I'll be looking into it myself. Probably will take me a few months as we're focusing on optimization right now, but I'll try to report back when I've accomplished something.
