You can't book something that isn't available through rci. They do have nightly stays if you have plus partners (bought at least one contract retail). If it is part of weeks inventory though you book the whole week or not at all. You don't have to stay the whole week but you pay the same either way and if you are skipping the front end you should call the resort to let them know when you will be arriving so they don't assume you are a no show.