You will need to open your extracted O_TEXT.AFS in GGS and find the ball you want to replace (they are the first 10 or so files in the O_TEXT). All ball files consist of two bins per ball. The actual ball bmp and then the mdl file.
The bmp is the picture of the ball and the mdl file lets the game know what shape the balls pattern is arranged in. You will need to replace both.
If you need a certain kind of mdl, then try this great site by WEman:
http://homepage.ntlworld.com/g.mercer1/index.html
He has all you can ask for in terms of balls and boots etc. Just remember to set the transparencies properly in GGS before you insert. Good luck!