New content is fairly easy to add to the game because you don't have to worry about overwriting existing items (which can be a bit more tricky).
For getting into the XML, I would suggest you go to CoreArmor.xml or CoreWeapons.xml and find something you think looks nice in-game. Then copy that <GameItemType> to a new .XML file you create in \Documents\My Games\FallenEnchantress\Mods\Yourmod\filename.xml.
Change the internalname of the gameitemtype (it's the unique identifier for the item). Then you are basically set to start editing.
Here's an example file you can extract to the /mods/ folder: https://dl.dropbox.com/u/32649007/DGB246.rar
I changed <IsAvailableForSovereignCustomization>0</IsAvailableForSovereignCustomization> in the original item to <IsAvailableForSovereignCustomization>1</IsAvailableForSovereignCustomization>. This lets the item show up in the "Create New Sovereign" which is a quick way to make sure the item is alright in-game.