thank you for your prompt reply and please excuse my slow feedback.
For those who are interested:
I could solve my Door Switch issue using the suggested M581 command and the extruder Extruder Endstop of E0 ("e0stop"). I have used it in a manner of triggering a "software" based emergency stop (https://duet3d.dozuki.com/Wiki/Connecting_an_Emergency_Stop)
M25 was not necessarry as M581 command now allows to directly trigger the pause print g-code by setting the T parameter to 1 (guess this only works since RepRapFirmware 3.01RC2 and later).
I'm using a optical endstop which hase a high edge when the door is closed.
Here is the code which I used and embedded into config.g => General preferences:
M950 J1 C"e0stop" ; Jnn = Input pin number (RRF 3.01RC2 and later only)/ C"name" Pin name(s) and optional inversion status.
M581 P1 T1 S0 C1 ; P1 = specifies input trigger (corresponds to M950 J1)/T1 = pause print/S0 = falling edge signal when door is opened/C1 only trigger when printing