Armor parts are stored as a mesh and a dummy. Internal player character systems load that dummy and mesh and place them in the tree structure based on another model. To use them as creature parts, they have to be attached to an actual creature model, and cannot be loaded the same as PC parts... unless VirusMan rewrites that part '>
What you need to do is get nwmax and find the part file you want to play with via nwnexplorer. Then get the creature model you want to put it on. Import the part onto the creature file and position it so that it is in the position of the creature corresponding part. Not just visually, but in pivot position and rotation of the 0-th frame.
Depending on your needs at that point, you can do one of two things.
The first thing you can do is click on the original part on the creature, choose attach, and then select the replacement part. Then switch to element selection mode and remove the original part mesh without deleting the whole object. This gives you the benefit of animation retention. You are basically copying the animation from the old mesh to the new mesh without having to do more work. I would suggest before you do this that you first copy the new texture onto the old part, so that there is no chance of multi-material issues.
The second thing you can do, but only if the model calls on another file for animation, is to simply put the part in position, as before. Then change the child/parent structure, replacing the old part with the new part. The new part must be named exactly as the old part for this to work after export.
Now depending on your need, you export it one of two ways.
If you did the first method because you wanted to keep animation in the new file, then you export the character WITH animations as a new appearance type.
If you did the second, you export it without animations.
A third option exists in which you do the second option the same as above. Then on your model base you set the "supermodel" designation to the original file. This calls the animations from the other file instead of using the ones on the current file. You can then export without animations and it will get them for you when you load it in the game.
It isn't something you can just do in the toolset unfortunately, so you'd have to upgrade your skills or have somebody do it for you. I bet there are a few who would.