The short answer is: any stand alone externalsensor (smoke, fire, water, etc.) that has a "NO - NC" output (normally open - normally closed) will work with the DS12 security Sensor. The NC contacts of the external sensor would connect to the internal input terminals (second input) of the DS12A. When the external sensor triggers, the NC contacts of the added sensor will open which will trigger an alarm signal from the DS12A. Look for add on sensors that provide selectable contact open or contact close upon trigger.
Also there are many hacking instructions on the forum for addapting DS12A and DS10A sensors to water and smoke detectors. Search the forum with terms like "DS10 (or DS12A)water leak detector" and "smoke detector" etc.
