Most sites say you can't convert pin lock to ball lock, but they never say why. Sheer speculation, but maybe the holes for pin lock plugs are larger, so you could drill out and re-tap a ball lock keg, but not go the other way. I don't see why you couldn't drill and tap extra holes and have both types on a keg.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk