You cannot modify a reservation online. You can book 7 nights online @11 months from check-in and then one week later, book the next 7 nights online as a separate reservation. Then call MS and ask them to merge those two reservations into one 14-night reservation.
I agree with Kat. Availability at SSR won't be a problem for your dates. You could wait a week and make a 14-night reservation online. If you are planning to book a tree house things might be different. I don't know how fast those might book up for your dates.