There’s a lot of detail about making your own, which you can skip.
A simple first step would be to make a small new module with armours you like.
IIRC Eurgiga’s armours are robes, which, when added to a part-based armour, change the appearance entirely while retaining the AC - nice and simple.
Modifying the OC is a long haul. You need to copy the .nwm files to your modules folder, renaming them as .mod so you can edit them in the toolset. Then you can change the armour item templates and update instances.
(I can’t remember whether item instances need updating, but I always do it for good order).
If you’re talking of a single armor part - like a chest model, it shouldn’t be too much of a hassle:
rename your new (part!) model to an exsisting one. (The internal naming within the model should also be changed)
rename the new texture (a PLT) accordingly
put your model and texture in the override or a HAK
If I see it right, Eurgiga’s version are all robe based, which means, you first need to create a (custom) armor in the toolset and - under Appearance - let it point to a robe model.
A robe model can replace a whole body! Easiest here would be to use a vanilla robe name and rename your model accordingly.
If you want to add new robes, they need to be added to the robe.2da.
The robe.2da also manages, which other body parts will be shown/hidden.
The robe order to choose from in the toolset is not determined by the numbering (eg. robe038), but by the AC-setting in the robe.2da (very confusing!). Eg. to make robe038, which has an AC value of 6.50, show up all the way at the beginning, give it a low AC value.
To preview/design a part in the toolset, you have to have a human model (=pmh0…/pfh0…) version of your part, because the toolset will use human base models to show the parts. So, if you want a custom armor for a male dwarf, you’d need a pmd0… version, so you’ll see it equipped on a dwarf model and a pmh0… version, so you can make it available as an item and design it (eg. colors) in the toolset.
Part based models use PLTs as a texture! The PLT must have the same name as the part model!