SpudFiles

Could anyone tell me how to make a tennis ball launcher with the body of a panitball gun (launches with compressed air).

if you are using a paintball gun and attaching a tennis ball barrel, i would not expect great results. the volume of air in a tennis ball barrel is MUCH larger than a paintball gun barrel and a tennis ball weights significantly more than a paintball

you could try it, but i doubt it would work very well

Are you trying to modify an actual Paintball gun?

It might be easier, cheaper, and more reliable if you bought a Palme regulator for your paintball tank. Then built something that looks like this
http://pukindogspaintball.com/cannons/BECC.htm
for a palmer regulator google search "palmer regulator" it will come up on top.
for ease of use you can skip the electric part and use a Husky blowgun to set of the moddified sprinkler valve.

Hope this helps. It will take some research on your part though to learn how to mod a sprinkler valve.
Good Luck and Welcome.