You're going to want to edit your torso model in a basic text editor.
Assuming you have the model in a decompiled format, it should open like a text file if you just drag/drop it into your text editor. If it's not decompiled, I would use NWN Explorer to find the original model again, hit the ascii tab, then right-click and "Export text" (leaving off the .txt on the end and just having it as .mdl).
Once in your text editor, do a find/replace to replace all instances of the original model and texture name with what you want for your new model name/texture. "Save as" with the new name.
For body parts (and other models), the "internal references" for textures must match the file names being used. For body parts, the textures have the same file names as the body parts (save the file type suffix).
Oh, and you're going to need inventory icon(s) for your new armor part. This icon should also be a plt, and the file name for that will likely be the same as the model file, but with an "i" at the front end (somebody correct me if I'm wrong on this part).
You should now be able to add your newly updated model, plt, and chest parts 2da, and inventory icon into your hak and save it.
If you haven't already, open your test module in the toolset, associate the new hak with it, and let it build (to add the hak contents). You can cancel the "build" if you want to cut it short, it won't hurt anything.
Hopefully your new armor part will be useable.
