Thank you for mentioning that you are in Israel. From this, we know that you will have nectar dearths (reduced nectar flow) and a winter season (even if it isn’t harsh). With that in mind, I would strongly recommend that you get a second brood box to put under the first before you add the Flow super. Any 8-frame Langstroth deep box would be fine, but if you want a matching box, you can order one from the Flow web site. Your colony is much more likely to survive the winter and periods when nectar is scarce if you have 2 boxes.
The general rule for adding space to a hive is to wait until the existing frames all have lots of drawn comb on them, and the comb is about 80% full of brood, honey or pollen. You want most of the comb to be covered with bees, because when you add a box, the colony now has a lot more space to keep warm and defend from robbers and pests. The need to have enough bees to do this.
So I would do the following in your situation:
- Leave the hive at one box for now, and inspect weekly to assess how much comb has been drawn and filled by the bees.
- Order (or buy locally) as second deep brood box.
- When the comb is fully drawn and 80% full, add the second brood box.
- When the second brood box is also fully drawn comb and 80% full, consider adding the Flow super if you are still expecting nectar flow this year. I live in California, USA, with a very similar climate to yours. I am not expecting to harvest my Flow hive this year - for the bees to succeed, it is better for them to build up their own stores first in at least 2 brood boxes. Next year, they should have a fantastic start in the spring, and maybe we will get some honey if we have enough rain!
Hope that helps.