Well, their UI is stored under \The Elder Scrolls Online\game\client\EsoUI\Ingame
There's an ingame.txt that contains all the lua and XML filenames + the path to them.
My guess is that it you have to add the path to the game directory, then add your addon to the ingame.txt and place a XML+lua file into your new addon folder. Something like that. I'm sure a bit of trial an error is enough to get it working.
The basic API and XML structure is inside their ui files.
E: Just to be sure: I'm just speculating. I don't know or tried anything.
