Trackballs can be very difficult to handle in terms of the electronix and signals. When we did the Wii trackball remote we were able to find the trackball used in blackberrys from Mouser.com, that track ball wasnt very traditional in the sense that it was just four switches that made contact when rolling the ball in the respective direction. with that track ball it would click click click when you rolled it so it was like a low frequency tapping of the direction when hooked up to the controller. Esentially that low frequency clicking was too slow in order to use the track ball in any real games, its sensitivity was not enough to make it a real usefull mod. So from that we would make a pic program that increased the sensitivity of the ball however each click would be much courser movement which gets rid of the sensitivity one desires in gaming.
The wii mote directional pad was well suited for that particular track ball as it was a digital input. The Analog controls on a 360 controller wont be as well suited as that track ball was really a digital input device. Doesnt mean it cant be done, just means you wont get any analog action from that trackball, it will either be full on or off.
If one was to attempt this mod I would suggest finding/hacking a track ball like a mouse that will give you signal that is proportional to the speed that the ball is rolling. I am not sure if a mouse will give you that signal without some hacking of the control electronix, but it is something you could investigate.
Remember also that the analog sticks are 2 potentiometers with spring returns, which means that when the anolog stick isnt being pressed the x-axis and y-axis signals will be in between +V and ground. For example if +V is 3v and gnd is 0v, when the stick isnt pushed in any direction the signal on the x and y axis will be 1.5V. When it is pressed up, the signal would be 3V on the y-axis and 1.5v on the x-axis. If it is pressed down the signals would be 0V on the y-axis and 1.5v on the x-axis. So more than likely a level shift in your signal or crafty ground connection will be required.
