With all your posts and input from @yveaux I combined it to the following.

2 buck converters, an arduino and some wiring.

The alarm will set 12v on PGY / PGX connectors if set and 0v if unset.

We only need to convert that 12v into an acceptable input voltage for ARDUINO digital in.
I used the buck converter for this purpose, 4,45v is sufficient to trigger digital 1.
and the 0v ... digital 0.
Don't use the internal pull up resistor of this pin ;-)

I tried powering the Arduino from the alarm as well with the separate buck converter... however that was nu success. Could not figure out why. Pins became unresponsive.

The arduino is now powered by separate power supply from the ISP modem...

And the final result:

.

I will post the code on my github. So everybody can have a look at it if they want.
https://github.com/sincze/jablotron_Mysensors_2.0.1

In addition I also added a doorsensor. Should somebody open the door I'm also notified.

And all thanks to MySensors.