A sprite is a sprite, there is no depth, so I don't follow what you are thinking of. If it were 3D then it might make some sense, but as you are working with flat 2D sprites all you can really do is 2D tricks such as blending, ADD, OR, XOR and so on.

If you want specific sprites to overlap over another then it's simply a matter of drawing order and you have to adjust that so that it is drawn last somehow. The use of a depth buffer seems like a waste of resources to do such things though.

...also, is this an iOS app I see? Very nice. Good work.