Their mainly 2 types Touch screens that are dominant,
1). Resistive touch screen : Cheap and easy to use. Used in low-price phones and gadgets. Also, widely used in Embedded Systems
2). Capacitive touch screen : Expensive, but excellent in sensitivity and somewhat difficult to use. Used in high-priced gadgets and phones.
Yes you can add touch screen to your Graphics LCD. You have to get the touch screen same in size as the Graphics LCD and fix the touch screen firmly on the Graphics LCD. And you have to add the interfacing logic of the touch screen to your program code.
If you're adding touch screen to your project, you must be knowing which kind of touch screen you have, because there are lots of touch screens available each having different configurations.
How to interface the newly added touch sensor to my controller?
I only know how to interface a 4-wire resistive touch screen. If you want to know how to do that, go to the link I've posted earlier. I've added sample code for LPC2148 controller.