The door sensor does not have built in support for adding another sensor.
However the Reed sensor is quite easy to get to and has large nice connectors, (see picture) so if you are somewhat handy with a soldering iron, you could easily add your own cable to it, and put it in parallel with the reed sensor, or replace it.
In regards to running the water sensor on AC, well you would need your own AC adapter that can deliver I suppose 3V, The sensor runs on 2xCR2032, but it seems to be able to run on only one.
There are two slots for the two batteries, if you connect a battery to the first terminal the sensors turns on, and you can run it like this. However connecting a battery to only the other slot does not work. So first I thought they were connected in parallel but then it should not matter what slot you use, and if they were connected in serial then it should not work with only one.