If you're booking at the 11 month window, IMO it's easier and better to call to add on nights. As long as you call before the 11 month window actually opens for the night(s) you want to add, you are almost guaranteed to get them. (Unless
DVC takes rooms out of inventory, the nights will be there because no one else can book "your" room before the 11 month window opens for the nights you want at the end of your reservation).
If you book the extra nights online, you will still compete with everyone else who wants that time and you have to go online every day when the window opens for each extra night and hope you get it. Then if you get all the nights, you'll have to call MS anyway to combine them all into one reservation. So better to call!
For example, let's say you want to book 12/10/2016 - 12/21/2016. On January 10, 2016, you go online and book 7 nights (12/10/2016 - 12/17/2016). You could call MS on 1/11/2016 and add the night of 12/17 and then on 1/12, call and add 12/18 and then etc. But since no one besides you can book 12/17 until 1/17, you don't have to call every day. My advice would be to call on 1/14 and add all the remaining nights on one call. (If you were calling every day, 1/14 is the day you could add on your last night of 12/20, so on 1/14, you could call MS and ask to have the nights of 12/17, 12/18, 12/19 & 12/20 added to your reservation, making one reservation 12/10 - 12/21 ).
HTH. Good luck. The trick is to get that first 7 night reservation!