best thing to do is drag a different object (I use datapads). Once the datapad shows up in the inventory, you can edit the ID to whatever the ID is of the item you wish to add.
here's a readme excerpt I have to my Hak Pad mod that demonstrates how to add objects to storefronts. However it should work for inventories as well as storefronts, placeables, etc.
Just open the .utc file rather than a .utm file in your case.
5. How To Add Custom Items To The Hak Pad's Storefronts:
First, you'll need Fred Tetra's KotOR Tool:
Now, once you start KotOR Tool, goto: File > Open GFF Files...
Depending on what you want to edit, open the appropriate file (.utm, .utc, .utp, etc.)
Once that opens, to your lower left is a button named "Inventory"...
~ Click It ~
Now here's the tricky stuff...
To your left is the game items, to your right is the store inventory.
Grab ANYTHING from the Left side of the screen (a datapad is usually
the easiest to distinguish) and drag-n-drop it over to the Right side
of the screen (The Inventory)...
Now scroll all the way to the bottom... and you'll see the item you
Click the "Dropable" check box so the check mark goes away
In the "ResRef" column, highlight the entry and type in the ResRef
(cheat code) of the modded item you want to be available. Next to that
entry, you can enter a quantity (how many of said item you want to be
able to buy in one store visit)... enter any number from 1-99
Now, check the "Dropable" box so that the check mark appears again...
** You don't have to enter a "Name", KotOR Tool will erase it anyways,
so there's no point in it
Now, click the "OK" button to the lower right, then on the smaller
window click the "Save" button and save either to your override folder
or wherever you opened the file from.
** Make sure when all is said and done, you drop the new edited and
saved file in your override...Hope That Helps!
if you want the npc to be wearing it, you should be able to right-click the body slot and edit the ResRef there.