How to create a custom function

I'm working on a blockset for simulink which should configure the periphery of a µC.

After generating c-code with the embedded coder, I get the two functions ..._initialize() and ..._step().

Now, I would like to create a custom function like:

void handler_sub(void){
...
}

This function should contain c-functions from blocks which are inside a subsystem of my model. The name of the subsystem is handler_sub. I need it to implement interrupts.

I will be grateful for any help!