Can an armed door be opened from outside an airliner?
This depends on the aircraft:

DC9, MD80, B727, B737

On some (especially older model) aircraft doors such as the Boeing 737 for instance this is directly dangerous, since the slide is attached on the inside by hand with the means of a girt bar to the aircraft. 

There is hence no way to open the door without the slide going off should it be armed. There is a helpful notice in red on the outside to "stand clear" when the red banner is crosses the window, the indication that it is armed. Judging by the space required and force the doors have, this is still most probably very dangerous idea that should not be used unless absolutely necessary.


Source

Airbus, B757, B767, B777, MD11, E-Jets

On more modern aircraft, this is safe: opening the door externally will push the arm handle to the disarmed position before proceeding to open it. This is for instance the case on all Airbus aircraft.


                
            

This video provides a nice example.