Placing a button on a touch screen?
It depends on the kind of touchscreen it is. At this point, I think most technology is based on capacitance, and not multi-layer resistance. If this is the case, you might want to look into how those touch-screen capable gloves work. These are gloves that have a finger and a thumb covered in some material that works with modern touchscreens.

There is also apocryphal evidence that a hotdog, cold out of the fridge, works on these screens. Make of that what you will.