I'm sorry, what you ask is not possible in MIDP 2.0. But there will be some other concepts which can implement your idea.
I have faced a similar circumstance. In the scenario,
I used TextFields instead of ChoiceGroups. I used label of the fields as the product name and the user input value as the quantity. So if the textField.getString() method returns "", it will be equal to an unselected Choice element.
Hope you'll get my point.