I just did this on the phone with MS yesterday, and it was quite easy. After it was confirmed that the reservation I wanted was available, someone from sales was conferenced in to sell me the points (5 in my case).
I was advised that I would not be allowed to purchase points again until my next use year, and that if I cancelled the reservation, the purchased points would behave as borrowed points and expire at the end of my current use year.
Overall, I was very pleased with the transaction. I am happy that Disney has allowed this option!
Melissa