You are going in the middle of Free Dining...making reservations for table service meals difficult, if not impossible, to get. You can still add the dining plan (you will have to pay for it, since the Free Dining offer is over), but I would try to get some reservations before you consider adding it. You may not be happy with your options.
I am arriving 9-25 during free dining and I have had my ADRs for months. However our party changed a bit and I had to change some ADRs, changing them meant changing others before I knew it my entire dining plan was out the window.
I was able to get just about everything I wanted including Chef Mickeys and breakfast at CRT.
Be flexible about dates and times and call daily and you might be surprised by what you get. If you try to get something popular like Chef Mickey's for a particular day and you only want it at a certian time, chances are you are going to be disappointed but if you make a list of what places you want in the order of importance. Then call and see if that place is available for any of the days you are there at any time. See how that works, as you fill in spaces in your plan it will more difficult but call daily.
In order to add the DDP your ressie, you must already have a "package" booked (a package consists of room ressie + at least a one day base park ticket). If you have a room only ressie, then you must cancel, rebook your package to include room, park tickets and the DDP. Just in case you were thinking that you could get in on the free dining promotion--it's too late. You will have to purchase the DDP and then risk being shut out of the most popular restaurants (and many of the not-so-popular restaurants, as well.
I am leaving on 9-26 and I just booked my ressies yesterday getting everything I wanted at pretty much the times I wanted to some very popular restaurants.
Free dining "ends" on 9/20 or 9/27(if you booked bounceback) so end of September you should be able to find adrs, but mid sept will be hard...lots of people are looking on the sept cancel thread, but doesn't seem to be a lot open.
I've been having some trouble getting some reservations for September for my trip. I have 4 and I'm just trying to figure out where to go next. I'm still hoping getting into table service is not going to be as bad as people are making it out to be when we get there. Making these reservations are making my trip so stressful! I hate having too much planned out before I get to Disney!!
I started planning our trip for the end of September about 2 weeks ago. I already have dining reservations made for our trip in December so that is very out of the ordinary for me but it was a surprise trip with a code I got in the mail! Anyway, I was able to call last week and get dinner at Kona and Breakfast at CP so I didn't feel too bad. I would have really liked Whispering Canyon and a couple other places but they were full. Be flexible!Good luck!!