If there is availability, you would be able to add to either end, and then drop the days from the other end of your vacation.
The only piece of this, I think, is that you have to have the points available to do it in order not to have to cancel and rebook.
For example, if you wanted to extend it, you would have to add Sat, Sun, Mon, and Tues first (effectively giving you 11 nights) and then drop the original Sat, Sun, Mon and Tues.
Or, you could do it day by day (add Sat, drop Sat), but that could be risky if you are going at a busy time of the year.
Good luck!