Zombies, Run! cannot connect with Fitbit in any way, and cannot use real-time step or speed data from Fitbit devices. The app requires a phone to run missions, so we access the phone's GPS and step count data to track your runs, so other devices such as Fitbit aren't required for us to track you properly.
We also have no plans to integrate further with Fitbit devices, or to develop a standalone app for them.