Expanding the chest without stuffing it up can be difficult or easy; depending on how patient one can be with it.
What you'll want to do is find some high-strength elastic bands, and "pin" or simply screw some short strips of them into gaps (that you will make) into the solid straps holding the lower front and back parts of the chest together.
Im simply lost for words on how to explain it properly, but i hope that helps, someone might have a photo of just that type of thing to help you out.