Body Mass Index (BMI) tells if your target weight or your current weight is in a healthy range when you are planning your diet.
The app calculates BMI using BMI formula published by Centers for Disease Control and Prevention. By definition, there is only one way to calculate BMI. Any "improved" BMI formulas, for example, taking into account muscle mass, are not BMI. They may provide better or worse quantification of your weight and body composition, but they are not BMI.
Thus, there is no way to override BMI calculation in the app. On the other hand, the app does not use BMI for planning and analysis calculations at all. BMI is calculated and displayed for your information only and does not affect anything.