I'm not sure if it's possible to completely create a new door, but you can re-texture doors, like in the picture.
First you need to locate the door you want to edit. The doors are in the segment folder, you need to locate the .fps file (door segment). Open that one and see what textures are being used and what .fpe file is being used for the actual door itself.
Now copy the .fps file as new file and the corresponding .fpe file (door entity) also. Locate the textures for this door, edit them and save them as new files. Now you need to edit the .fps file to point to the correct texture and .fpe (door entity). Also edit the .fpe file to point to the correct texture.
I've used the Door Armoury (use).fps door as an example for you (it's in the segments\scifi\scenery folder, the parts in bold you need to edit:
;Segment Spec
;Header
partmax = 2
lastmeshmax = 0
;Segment Parts
partmode0 = 0
meshname0 = meshbank\scifi\scenery\doors\door_a\door_a_frame.x
offx0 = 0
offy0 = 0
offz0 = 50
rotx0 = 0
roty0 = 0
rotz0 = 0
texture0 =
texturebank\scifi\scenery\doors\door_a\door_a_d2.tga
textured0 = texturebank\scifi\scenery\doors\door_a\door_a_D.tga
texturen0 = texturebank\scifi\scenery\doors\door_a\door_a_N.tga
textures0 = texturebank\scifi\scenery\doors\door_a\door_a_S.tgatransparency0 = 0
effect0 = effectbank\bump\bump.fx
colmode0 = 0
csgmesh0 =
csgmode0 = 0
csgimmune0 = 1
lightmode0 = 0
multimeshmode0 = 0
materialindex0 = 3
partmode1 = 3
meshname1 = entitybank\scifi\scenery\doors\door_a_u.fpe
offx1 = 0
offy1 = 0
offz1 = 50
rotx1 = 0
roty1 = 0
rotz1 = 0
texture1 =
textured1 =
texturen1 =
textures1 =
transparency1 = 0
effect1 =
colmode1 = 0
The last bold part tells you where to find the correct door entity for this door segment, you need to locate that one and edit it also as explained before.
