Not sure what you mean by different pool of points:
1. If they are points from a different use year, you can go online (or call MS) and make the reservation for the one night with those points and then contact MS either by phone or email and have it link the two reservations from the different use years, which will assure that you will not have to change rooms..
2. If they are points you own in the same use year as the original reservation but are from a different resort, then to accomplish the same result:
(a) if the original reservation was made during the 7-month window, you can just go online and modify your existing reservation to the new one
(b) if the original reservation was made during the 11-month window, AND the new total reservation time for the same room is shown online as still being separately available from your reservation, you can go online (or call MS) and modify the existing reservation to the new one.
(c) if the original reservation was made during the 11-month window, AND the new total reservation time for the same room is shown online as not being avaialble for any of the nights, your options are to either call MS and have it add the day to your reservation, or first make the one-night reservation online and then contact MS to have the two reservations merged or linked, whichever MS will do.
3. If the different pool of points are points that have been transferred to you, then regardless of when you made the reservation, your only option is to call MS and have it do whatever is necessary to make or change your reservation and assure the two time periods are combined into one reservation.