Hello Polanski welcome,
Where exaclty do you want to add a door? As Flo wrote there could be some problems with collisions.
I recently imported a door on Libya3 mission.
It was not that complicated but it helps a lot if you have 3ds Max (64-bit) and hdmaster's plugin installed. Then positioning goes a lot faster..
As there was no door in the first place i've edited scene.4ds, added the door mesh and it's associated BMP. I've just copied the cell door from Libya1 (downstairs, where the prisoners are that you have to free) It's called HV01_174 and the bmp: G_AF6_018.BMP
If you don't know about the 4ds format,
infos can be found here
.
Each image has an ID. First image id1, second id2 and so on. Always add new bmp at the end of the image section otherwise all you ids are getting messed up.
Your new mesh object then needs that image ID so it load the correct image on your mesh.
Hmm i wouldn't understand myself if i read this..
I've uploaded my edited and the original Libya3 scene.4ds so you can simply compare them with any Hex Editor.
Well just ask if there are further questions.
There's an easier way as there's already a door model existing:
l_door.4ds
Add a new model in your bin file, add the door definition and that's it.
In the definition you can make several adjustments on door behaviour,
have a look here
.