If this is for a single machine... I'm not sure I'ld even change the name of the pins. I might just make sure the number of servo's is less than 2 so it is safe to use those pins.
And then I would put the extra sensor on SERVO2_PIN. I would add the extra logic at the same places the current logic accesses SERVO3_PIN for filament runout.
And then I would put the extra sensor on SERVO2_PIN. I would add the extra logic at the same places the current logic accesses SERVO3_PIN for filament runout.