I am assuming that you want this to happen when you edit the field or if you have a field set up to allow adding new choices when editing a post.
When ACF updates a field’s settings, just before saving the field to the DB it calls
$field = apply_filters( "acf/update_field", $field );
It is possible to set up a filter on this hook that will do what you want. In the filter you would need to check to make sure it’s the right field by looking at $field['key'] and then you’d need to create the logic to sort $field['choices'].
