Say you want to duplicate chest 073.
Find a model number in parts_chest.2da that is not in use (indicated by ***** in the AC column). Let's say 199.
Extract all the models p??073.mdl, where ?? is the gender and race code. Decompile if necessary.
Use NWN Model Renamer to create the new series of models p??199,mdl.
Copy all the textures ip?_chest073.plt and p??073.plt to *199.*. Don't use the model renamer for this.
Finally, amend line 199 in parts_chest.2da to the desired AC.
This is much simpler than it probably sounds.