That can't happen. You can't open the door of a large jet in flight even when the aircraft is unpressurised - the slip-stream will hold it closed.
The 747-400 has a procedure for opening two main deck doors during flight to allow for smoke evacuation (thats why there are smoke evacuation door handle straps carried) This procedure is carriede out by cabin crew under the direction of the captain once the aircraft has depressurised at a safe altitude.
It also has a procedure for cabin crew to guard the upperdeck doors when the automatic door lock fails on gear up (as if they dont automatically lock the doors can be opened until aircraft is at altitude and fully pressurised).
What does stop the aircraft doors being opened once pressurised is that most large aircraft doors are 'plug' type doors. That means they are actually larger than the whole they fit into (this happens through door panels expanding the door size when closed (Airbus), or the doors having to come into the aircraft then push into the holes (Boeing). Air pressure differences stop these 'plugs' from being removed whilst the aircraft is pressurised.
