Extra PWM channel

Hi all,
In addition to the 4ESC I need to drive an additional servo motor using a PWM, using an Erle-Brain3. I tried to read the apm code, module by module from its source:

I figured it must be in one of these files. Does anyone know which one and How to use it?

Can you get away with using the mavlink DO_SET_SERVO command?

ServoN_function might be worth looking into