Salan has, but I haven't
I've never added an additional item, so I can't really help you there :/
Modding is a lot of trial and error, and there are only few resources available..
Some tutorials are available here, but they don't cover everything : http://docs.runicgames.com/wiki/Main_Page
However, I would say that you need to add the item to the item list (under Items on GUTS maybe ?), then you need to make it spawn somewhere
There might be a way to make it available to buy in a merchant, but I don't know how.
The only way I would know how to do, is by creating a new spawnclass, and assigning a monster to loot that spawnclass.
Spawnclasses are kind of group of stuff (monsters, items, you name it) that gets used here and there. The treasures that you loot in chests are spawnclasses for example.
Each monster has one or several loot spawnclass - these are the spawnclasses that gets called whenever the monster dies. For example, Grell has an "eyechance spawnclass something" and a "champion loot spawnclass" or something like that.
If you add a spawnclass of your own creation, with your item in it, to one of the monsters in-game, then that monsters will be able to drop your item.