If you need to use a different IMEI number to that of your phone because any kind of application requires you to have a new one, you should download IMEI Changer. This app is capable of generating a valid number in a matter of seconds.
Xposed IMEI Changer or how to change your IMEI number
The thing is that to make this app work correctly, you're going to need to have a rooted device (with superuser permissions, as you already know) with Xposed framework installed on it. From there on, you'll be able to modify your IMEI, which is basically the code that identifies your device all over the world.
The main screen shows your original number, the device's current number, and the possibility to generate a new one. By simply generating it and pressing the Apply button, your device will adopt that new number. The good thing about this app is that it doesn't generate random numbers but is capable of finding totally valid and operative IMEI numbers.