Yes, you can absolutely use the Kahunas app in the meantime while your branded app is being developed 
You’ll be able to set everything up as usual, program workouts and nutrition plans, create forms, set up automations, and even add your clients to the Kahunas app.
Once your branded app goes live, your clients can simply log in using the same credentials, and all their progress, data, and programmes will be carried over seamlessly 