Smoke detectors are on dedicated circuit, how can I add a light to that circuit?
Adding anything is against code in most jurisdictions.  They want wired smoke detectors to be certain of working.

Edit: 3PhaseEel points out that having smoke detectors on an unswitched branch circuit is allowed.  See his comment below.

Good wired detectors still have a battery and they chirp annoyingly when they are without power.  And will do so for weeks.