So, now I have completed (and refactored) the NexaSmokeAlarm-item. I added the ability to actually trigger the alarm via the NexaSmokeAlarm-item too. I also added a "self learn"-function for the address of the smoke detector. Each detector have its own unique address, and by activating the "learn mode" on the home item and then manually triggering the alarm by pushing the test button on the smoke detector, the NexaSmokeAlarm-item learns the address from the alarm.
The protocol decoder/encoder and the NexaSmokeAlarm-item are available in the nightly build. One cation - never rely on NetHomeServer for performing any real fire alarm functions. The smoke detectors provide the real alarm, the NetHomeServer can simply provide additional features when the alarm goes off.