I was going to suggest the same thing. I have a small contract and the weekend nights eat vacation points - I will be paying cash for an upcoming Sat night at OKW (it is just over $300 for a 1 bedroom). The same night uses 54 points - if I transferred 54 points it would likely cost me $540 if I paid $10 a point. If I added on thru Disney it would cost roughly $5000 for those points. Eventually, I may decide that an add on makes sense, but for now I am using cash nights.
Also, my understanding is that cash/point stays are linked as one stay and will be treated as such (as long as you remember to have MS link the stays). You do not have to change rooms and you check in just one time. Your trash & towel service and housekeeping schedule is the same as if your entire stay was booked on points - and, if you purchase
DDP, you must purchase for all the nights of your stay, not just the points nights (or vice versa).