Only two alternative ways to do the reservation when using two use-years:
1. Transfer points from one use-year to the other and then make the reservation.
2. Make two separate reservations, one 11-months out from your arrival date reserving the beginning of your trip, and the other 11-months out from the remainder of your trip using the second use-year. You then contact MS and have it link the two reservations to assure not having to change rooms.
Member services cannot (and will not) just add days to a reservation with points from a different use-year absent first doing a transfer of points from one use year to the other.