Study the asset pack object definitions; decide what you need for your wallset. The bare minimum will be a pillar, floor, ceiling, and wall.
Doors, secret Doors, pits, ceiling shafts, and separate stairways for Up & Down will make the wallset more versatile. Examples of all of these are found in the asset pack. To begin, you can try modifying the official asset models, and saving them as new models. The files must be saved as .model format files (which both the GMT & Blender w/ Bitcopy export can do), but in the object definition you must write the filename extension as .fbx; so the file "mod_assets/models/my_wall_01.model" would be written as "mod_assets/models/my_wall_01.fbx".
It's possible to script new interactive wallset objects. Check out the One Room Round Robin 2 LoG Mod.

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If modeling (from scratch) is too much to begin with, then you can use Blender or the GMT to replace just the material textures of the models.