drusba is correct. However IMO the current rules are somewhat "absurd" if your April reservation is still available, or was made inside the 7 month mark. Take a closer look at the very clever user name "drusba" backward.
Assuming your April reservation was made inside the 7 month window, you could have accomplished what you wanted had you done the following: Prior to making you reservation for April, you could have made an unneeded reservation for anytime in 2016 that used the same number of "regular" points that you will need for your 11 month reservation in December (As long as that unneeded reservation was made with points that could also later be used for your December reservation). Then when you made your reservation for April, you would have needed (and been able to) purchase one time use points to complete that reservation. After you secured the April reservation, with the one time use points, you could have then cancelled the unneeded reservation and had the "regular" 11 month points from that cancelled reservation available for your December reservation. (Cancelling a reservation does not reverse the previous purchase of one time use points)
You could still effectively do the above, if the dates for your April reservation are still available. You would have to cancel your April reservation, make an unneeded reservation with regular points, for the amount of points you need for the December reservation, and then rebook your April reservation, which would require purchasing 7 one time use points for that April reservation. Then you could cancel the unneeded reservation, and those regular points would be available for your December reservation. Obviously, you could not do that if the April reservation is not currently available.
ETA: If you have points you could borrow for the December reservation, that would be simpler and IMO better, especially for only 7 points. I only outlined the above to show what could be done.
2nd edit: Based on information in the posts below, the following I had added in my first edit appears to be incorrect.
Also, I believe you can only purchase one time use points once per use year, so if you did purchase them for your April reservation, you might not be able to purchase them again for your December reservation if it turned out you needed more points than you expected for that December reservation.