Currently this cannot be done with gE, and nothing has been said development side to indicate it will be possible anytime soon (though we have put in the request several times). It also seems like the next step for mobile support.
That would be great if it did do this. From what I have been told by 3 iphone developers is that they make 100 times more money with micro transactions then just selling the game. I am aware that this might not be the case for all game but I am sure you would at least double your income with this.