how do u make different accessories?

Can you make accessories without changing anything else about the sim? And is there a limited number of kinds of accessories.

For example, could I make nipple rings or belly button rings that the sim would wear as part of a swimwear outfit, and how would you make it so the nude sim underneath stays intact.

Edit:

Basically, I am trying to figure out how to make a "skin" that lies overtop of a mesh without actually changing the mesh. I am thinking the accessories like sunglasses, for example, will have a way of doing this. I don't know if I am explaining this correctly. It seems to me that some of the accessories are separate from the mesh. Is it possible to make an accessory without making a mesh with it. If so, it may be possible to make a T-shirt and call it an accessory. Then we would be able to have the sim change "accessories" without changing the mesh for each outfit, thus eliminating the problem of the sim changing body shape with each outfit change. Does this make sense to anyone?

I was looking at a site with a tutorial explaining how to make the hairstyle change with the formal outfit, so you could link an outfit category with the bodypart that was going to change. The formal category was linked to the hair bodypart. Other bodyparts included fullbody, top, bottom, face, and ears. I was thinking, what if you could 'unlink' the formal category from the fullbody bodypart, so nothing in the mesh would change, then link it to an insignificant bodypart such as ears and made a skin that covered the fullbody without changing the mesh. Then you could have the sim change outfits and the "ears" change unnoticably and the skin would change, but the full body wouldn't, so the sim would retain the same bodyshape through every outfit category.