After being in the South Berkeley area with close to 15 years of beekeeping, in Summer or Fall I would not try and boost bee populations in any of my hives.
Why? The dearth is in full swing and most years I don't see more honey until perhaps November, so I let the bees manage their own population. Some years I have systematically weighed hives through the summer and fall and after May or June I don't see weight gain, or perhaps a slow weight loss, so I can see the bees are just maintaining the honey already in the hive. Adding more bees just means they have more bees that die off without any real effect. Even if there is another nectar flow in the Fall, it is small and doesn't kick in until October or November, i.e several months from now. Now very small hives (
(i.e. just a few frames of bees) can happen from late splits, late swarms (this is rare as I manage swarming) or they just haven't built up well from the spring. Hives that didn't build up well I requeen. Late splits I mostly use for requeening or I leave alone to sell in the following Spring once their temperament and vigor is proven. They seem to grow even in the dearth while the bigger hives seem to just maintain themselves.
If you don't manage mites, some hives will dwindle and die in the winter. Adding bees won't help these hives. I manage mites, so I mostly don't see winter deadouts.
Some years I have taken swarms in August. One year I had an August swarm starve and collapse, so if you take a late swarm watch it for resources and if necessary steal a honey frame from another hive.