You could probably take out the bullet the pistol created inside itself and put in your own in its place, but when you fire it it will certainly recreate its normal ammo, not the type you inserted.
ah ok thats a shame and it would be very expensive to add ghost touch to the pistol itself since my gm said he'd let me add a +1 ability to it for 22k more but a +3 bonus like ghost touch would be insanely expensive lol.