It sounds like you are talking about a point transfer rather than a rental. If you arrange for, say, a BWV owner to transfer BWV points into your account, you can use them to book BWV 11 months out. If you want to add additional nights to that reservation, you either need more BWV points to book those extra nights at 11 months out or you must wait until 7 months out from those additional nights to use non-BWV points, subject to availability at that time.
Renting points is when you arrange with a
DVC owner to have that owner book a room for you with his/her own points and then you pay them for that reservation. The other owner owns and controls the reservation. You cannot add nights to it using your own points. You can however book the extra nights you need as a separate reservation using your own points at either 11 months out (if you are using home resort points) or at 7 months out (if using non-home points). You can then ask Member Services to link the two individual reservations so that the room assigner will put you in the same room for both reservations so you don't have to change rooms.