This is actually part of the next big update (barring any unforeseen problems). It's called "props" and will allow people to load in 3d objects and place them anywhere onto a character. There will be a selection of objects already available, but people with modeling skills will also be able to create and distribute their own. So yeah you could use it to make shoes, jewelry, horns, ears, tails, spikes, whatever.
Making a cat girl and an elf girl were the original ideas and then I realized it would be cool to make
any kind of girl, guy, or whatever. This is one of the major add-ons I've been excited about getting into the sim for some time now.