Why don't you find out? If you have an old fan you are not too worried about damaging, try reversing the polarity on the input wires and see what happens. Just use a small 9 V battery and touch the wires briefly. (You can guess from this that I don't think it will work and a small 9 V battery will minimize the risk of letting the magic smoke out.)
Also, the fan blades are curved and are designed to move air in one direction only. If you examine them closely you can see this. If you did manage to make the fan run in reverse it wouldn't move air very well.