Hello.
The Zumo shield does have free pins that you can use for connecting other sensors (even with the Zumo reflectance array attached). There are a lot of different combinations of features that could result in different pins being available. You might find this post by Jon helpful. As his post mentions, the “Arduino pin assignment table” section of the Zumo user’s guide is a good way of seeing what the different pins are used for. Additional information about the pins used by the Zumo reflectance array can be found in the “Adding a Zumo reflectance sensor array (optional)” section of the user’s guide. By comparing this information with the “Front expansion” section, you should be able to determine where the connections can be made.
Also, it looks like the picture you referenced came from Pentura Labs’s Blog. Their "It Can See; Giving Your Bot Sight!"post details which parts and pins they used in their project.
-Brandon