Right now you can make new textures for shoes, but not actual new shoe models. I'll have some new models available at some point, and there's a chance that I can add the ability for users to create their own models but I need to figure out how to make that work seamlessly before I introduce it.
I'll write a tutorial (maybe later today) on how to make new shoe textures but in the mean time if you want to play around with it take a look in the \Sex Sim\Runtime\_aData\shoes\ folder to see how they are done and then use the same structure for your own but put them in the \Sex Sim\Runtime\_user_created\shoes\ folder so they show up.