You can add a standard 8 or 10 frame Langstroth box to your Flow hive, depending on what size of Flow hive you bought. If you bought a Western Red Cedar (WRC) Flow hive, the wood may not match in color, as it is hard to find WRC Langstroth boxes.
Be aware that the boxes from another supplier may be slightly different in size, but not enough for it to matter to the bees. This is because the dimensions of Langstroth boxes can vary by up to 1/4", depending on which pattern the manufacturer decided to use. I just center them, so that there is an 1/8" overlap on each side. From a distance, it really isn’t noticeable.