Adding equipment means the items an Enemy can be wearing when encountered. In other words, is a list of stuff that a certain enemy has access to and the game picks one from each category at random. Right now Item addition is something I have not checked, BUT will be checking soon. I just finished going through the whole Pony Race Mod and figured out how the heck you add new races, which let's be 100% transparent, should be fairly straight forward but I have yet to code it, so I won't be jumping the gun for now and saying it's "Easy".
In terms of New Items, AFAIK, normal items are stored in a database, that includes armor, accessories and weapons too, so I SHOULD be able to do it pretty much the same way I'm doing it now with other stuff... BUT, and it is an even bigger but that caps give it credit: Armor, accessories, weapons and potions have special effects, that are linked to other scripts outside of the item database, so implementing them PROPERLY is going to require some digging around in the code.
Aric Triton probably knows more about the item and gear system of the game right now.