Can I add a light fixture (with a switch) to an existing switch/outlet circuit?
You'll need to tap power at the receptacle box

Since power is coming in at the receptacle end of this circuit according to your diagram, you'll need to tap power to the new switch and light there, as that's the only place in this you'll find always-hot and neutral -- any point from the existing outlet to the existing switch won't have neutral, just always-hot and switched-hot.