Okay, first of all, you want to make the door "open" like someone pressed the handle down.
Use this code:
SetSwingDoorClosed("doorname", false, true);
(Replace doorname with your own doors name.
Secondly, you want to make sure that the door does not close by itself instantly after the "handle is pulled". I think it like someone is holding the handle down. Use this code for that:
SetSwingDoorDisableAutoClose("doorname", true);
Now you want to push the door open, but in order not to push it in the wrong direction, check in the level editor which axis you're supposed to use, and also if it's a negative or a positive value. (It's probably the X or Z axis unless the door is on the ceiling or floor)
Now apply the actual force to open the door:
AddPropImpulse("doorname", float afX, float afY, float afZ, "");
Replace the "float afX/Y/Z" with the amount of force you'd like, and make sure it's the correct axis.
Creator of The Dark Treasure.