You cannot pay a combination of points and cash for the same night. You can pay for some nights on points and then pay for addditonal nights with cash (through CRO, subject to availability). If you do this, you may have to check out and then check in again. However, most report that MS can cross reference the CRO cash reservations and that if you mention it at check in, you can usually stay in the same room.
Sometimes, Members can get rooms for cash with a 25% discount off the rack rate. You would reserve these through MS, not CRO. These rooms either come from developer inventory or from points inventory that MS doesn't think will be reserved using points. These type of discounts are usually not very plentiful for the sold out resorts and are most likley to be available less than 60 days ahead of arrival. Sometimes, CRO does have better discounts (AP, codes), but you really have to check to be sure. HTH.