To add nights online, you would need to wait until 7 months out from the first night you are adding because you would be making a new reservation for the additional nights (and then email MS to link the reservations. You can call and add a night beginning the day after you set up your initial 7 night reservation done at 11 months out.
As to making an Easter time reservation at 7 months, the advisable course may well be to call daily and add nights rather than waiting any time to get the additonal nights. There are many things open 7 months out during Easter time. However, there is a progression to the rooms filling and if the nights you have to add are nights right after Easter, you should likely be calling daily to add them right after making the initial reservation. What I have seen at Easter time is that come 7 months out from Easter Sunday a lot of the near-parks still have openings such as BWV pool/garden, BLT lake view, and even possibly BCV. However, at 7 months out from the Monday, Tuesday, Wedneday or Thursday night of Easter week, you can find those nights filled for one or more of the those near park resorts, particularly for studios or 2BR lock-offs. In other words they are being filled by members reserving 7 months out from Easter Sunday.