Make a folder in the reload entitybank folder called SegConvert.
Copy all the x files and textures into this folder. _d,_n,_i
Copy any one of the fpe file in the Fixtures folder to the SegConvert folder.
Make copy of this and rename it one of the x files, start with the floor to make your life easy. Open in in a text editor and change the model name the detail name and the texture. Save the file and try it in reloaded.
Notice its position and if it imports to where you expect. If it does go to the next piece. The wall section. Do the same for the corner and the ceiling, till your have all the pieces in the map editor where you can assemble them.
That should be fun. repetitive, but could be done. (not in my opinion)
I would just model a Building at a one to one scale and then export whole wall sections into the map editor. Works great plus you can modify the meshes as you feel to get a custom wall. texture it and export a x file and change the info in the fpe file as above.
A little tip. Make your model around the 0,0,0 point. When you export them out in their position they will import into the map editor at the proper spots when you use the BB keys in the editor.
Anyway have fun in which ever way you choose.