The retrofit kit only allows Normally Closed devices like window and door contacts. I’ve read that other IOT kits allow both NC and NO devices switchable through software. If you allowed Normally Open devices then hardwired smoke detectors or motion detectors could be connected through a relay such as the First Alert RM4 or the Kiddie sm120x. For people with large homes and, for example 4 smoke detectors and 3 motion sensor, this is a large investment we want to preserve and this feature would bring new customers to Ring.
Actually I want to clarify my suggestion - the RM4 or sm120x should allow connection of interconnected 120V smokes to the existing retrofit kit. But if the retrofit kit allowed Normally Open contacts then some smoke detectors such as the ITI 449 which are designed specifically for alarm systems would work without modification, as well as motion sensors. These also require power which can be provided by the existing alarm system.