Yes, VIN is the +5V pin that’s internally wired to the +5V pin on the circuit. In older version of the enclosure, it’s marked VIN; in newer version it’s marked +5V. This pin is normally used to power certain sensors that require +5V. But it can also be used to provide +5V to the controller. You have to be certain that it should be regulated +5V (not to exceed 5.5V).
If you want to make sure current can only flow in one direction (e.g. from external battery into +5V pin and not the reverse), you can put a Schottky diode inline to control the direction.