To add a furniture you would create a spritesheet containing all the images for that furni and a javascript file with some definitions of where to find the sprites, when and where to draw them (examples of that js file can be found a bit back in the thread)
As for clothing there's no way to do anything with that yet. It's currently giving users a random figure out of 10 pregenerated spritesheets (script for the spritesheet generation can also be found further back in the thread.
Could you later on the process give us an example on how to add furniture with a handy dandy tutorial? And regarding to the clothing, does this mean people can only have preselected clothing combos? Or can they actually change their looks shoes, for example? If so, does this get changed any time in the future?