I'm not a Z21 user or use ESU control but as I understand it the ESU mobil control II is an Android device with added rotary control.
ESU supply (or will supply, I'm not sure it is available yet) an app that will talk to their command station to run on it. Since it is an Android device you could download any Android app to it and run it. You could download the Z21 Android app and that should enable you to talk with the Z21 command station.
HOWEVER since the rotary control is not something that is standard on Android I suspect that you would NOT be able to use the rotary control with the Z21 app. To use the rotary control you would need an app that was specially written to use it, as far as I know there is no such app available. I would be glad to be proved wrong if anyone has any other information about this.