I rule that a baseball is a thrown weapon, mostly because if it's improvised then it's subject to Accuracy 3 (per Street Grimoire, if you're playing 5E) which is ridiculous for an aerodynamic object which is designed to be thrown. It's not a beer bottle or a credstick or a wrench or something else awkward.
Str(P) as the base damage is a good starting point, and an adept would get +1 from Missile Mastery on top of that. I use the adept's Physical Limit as the Accuracy.