You can call the Marriott Rewards reservation line and use partial points, as long as you have enough points for whole days. I just reserved the Residence Inn in Chicago, 2 nights on reward points and 2 nights paid, but only by using the reservation line.
As long as you have enough points to cover a day, you can make two reservations, one using your rewards points and one to be paid for. When you check in at the hotel, you can request that you keep the same room for the duration of your stay. I have done this several times.