I had a rez last week for Wednesday/Thursday nights and then needed to add Monday/Tuesday on to it after the fact. I had no problems making the second reservation (Diamond force, but online) and then just telling the folks at the front desk that I'd like to stay in my room. I acutally had one of three executive suites at the ES Tampa Downtown/Convention Center and they still let me stay in it, though they originally planned on giving it to someone else, as I described in this thread.

Occasionally I've had issues with the rate not "changing" during my stay, despite the less expensive rate being available for part of it, and just booked the multiple reservations and never had issues with it. So back-to-back may be frowned upon, but it is certainly possible, and easy to do online, generally speaking.