But can I activate Kayle’s empower ability even if she’s already empowered?

Yes. Kayle’s empower ability can be activated any number of times. If she has less than 3 empowered statuses, she will become empowered. If she has 3 empowered statuses, the ability can still be activated but nothing will happen when the ability resolves.

Related: Rule 002, Kayle, Justified