Add functionality to existing Android and iOS app (with server side program and database).

I have an Android and iOS app that tracks an user steps with Google Fit / Apple Health API, and it rewards the user with a coin based on the number of steps the user has gotten. It also uses a backend simple program to get and publish certain data to the app. I need to add functionality to both apps. I need to add an option to pick a sport (from a list that the app should show), and let the user start and stop a timer. When the user stops the timer, and based on the sport the user had chosen, it will give him a number of coins.