Please don’t add the Flow super at this time of year. I would definitely order another brood box, but your local beekeepers are the best source of information about whether to add it right now or not. My intuition would be that you may be too late. If you do add it, I would suggest adding it below the existing brood box, then take it off in November if the bees are not using it. You can add it back again (below) in Spring when the colony starts to build for the next season. It is best to have plenty of bees in the space available, rather than too few to defend the hive from pests.
I like to add new brood boxes below for the following reasons:
- Less thermal stress for the bees. Warm air rises. The bees generate heat to keep the brood (or the cluster) warm, if you put empty space above the brood, it makes more work for them.
- Less rearranging for the bees. Bees in natural cavities tend to build down. If you add space above, they usually move everything up, totally disrupting their current hive arrangement for several weeks or more. If you add space below, there is much less tendency to do this.
The Flow hive was developed in subtropical Australia, where one brood box can be enough. Also, many commercial beekeepers run on one brood box. However, for hobby beekeepers, you are much more likely to over-winter successfully if you have two brood boxes and in some climates, even three.
So for now, I suggest you get local advice. I wish you all the best!