We plan on doing just what you said and add on a day before and after to have more park time. I'm guessing we will need to just buy our own separate tickets for those extra days.
I did the China ABD this June/July and the heat/humidity was awful. Being from the south, I didn’t think it would be an issue but there was no way to anticipate how different it would be. The one thing that made it bearable was that I only packed moisture wicking clothing for our daily activities based on the recommendations in previous trip reports.
Because I wanted to make sure I could do everything I wanted to in the parks, I did a pre-day at HKDL and a post day at Shanghai DL. The guides said that our tour group ended up getting to HKDL earlier than other groups, so we had a little more time in the park. However, most of us were so tired that we didn’t ride many rides. I made sure I used all the fast passes ABD provided. Because I had been there the day before, I didn’t feel the need to rush while in the park. It also rained while we were there, so make sure you always keep the ponchos you’re given with you. They also came in handy for riding Roaring Rapids and I was surprised by how many locals had them on for the ride. I went online and bought my HKDL ticket before I left the US. I also downloaded the app, but didn’t find it that helpful while in the park. I took the subway to HKDL on my pre-day and it was an easy trip.
Unfortunately, my post day at Shanghai DL was less enjoyable due to the over 2 hour wait for most rides. I bought my ticket online before leaving home (you have to give them your passport number and bring your passport to the park to retrieve your ticket). I had planned to purchase fast passes in the app once I got to the park, but they were all sold out. I draw the line at standing in line for more than 30 minutes for a ride even at Disney. Therefore, I only rode a couple of rides the second day and went back to the hotel. I second the recommendation from
@Poohlie that you get a package that includes fast passes. Without them, you won’t see much at Shanghai DL. I took the subway from the Peninsula Shanghai to DL. It took an hour, but was super easy thanks to Joe telling me how to get to the station. I also downloaded their subway app which made getting around super easy.