This tutorial explains how to get import a piece of armor into Morrowind. We'll focus on a helmet, and suppose that you've already created a helmet NIF file and an inventory icon image. Other pieces work similar.
Create a Helmet Body Part
Create a new body part, of the type "Hair". Yes, open helmets are "Hair": they replace the hair. Closed helmets should be "Head": they replace the head. Right-click anywhere in the body part table to start the pop-up menu. Select "New".
Name the Hair part "a_amorilia_helmet" (armor parts start with "a_"), select Armor as type, make sure it is playable, and add the newly created "myhelmet.nif" as art file (this is the model used to show the helmet when equipped).
Create the Armor
Now create new armor. Right-click anywhere in the Armor table, again select "New".
Give the helmet ID "amorilia_helmet", name it "Amorilia's Helmet" (this is the name shown in the game), adapt the weight, healt, rating, and value, and again add "myhelmet.nif" as an art file (this is the model used to show the helmet when not equipped).
Select our newly created icon as inventory image.
Select "Hair" (or "Head", if you had a closed helmet) as biped object, and "a_amorilia_helmet" as male armor.
Put It in the Game & Test It Out
Now drag our new helmet onto the dwarf chair in Arrille's Tradehouse. Line it up with a chair, or wherever you want to put it.
Save the plugin, lauch the game, and check out your new helmet in Arrille's Tradehouse.