[MDL ED] A door with other material

[WALP]

I dont think its that simple, since the texture that is being used is usally pointed at from the collada file, and the texture is often also what defines which material that is being used.
If I were you I would copy all the door files except the mat., then open the collada (dae.) with notepad.
Find "library_images" and change it so it points the texture file the material will be using for color(also known as the diffuse texture), see the tutorial on retexturing for more detail:

http://www.frictionalgames.com/forum/thread-17299.html