Simulink Engine is designed to work with Signals which functions of time and the available Integrator blocks calculate integrals with respect to time. So, if you are planning to use this integration operation on a signal, I'm afraid you'll have to perform some calculus to convert the equation as an integral with respect to time.
If however, this integration is just an operation used in your model and the quantity 'P' is not an input signal, you could use 'MATLAB Function Block' to calculate the integral in MATLAB code and use the result in your Simulink model.
Hope this helps!
