I was going to suggest doing it by neighborhood, ward, or subdivision by way of mapping the buildings around the outside of the area with zip codes and then filling in the rest inside of the area under the assumption that they are the same. I’ve done it that way a few times myself and while it’s not as efficient as just importing the whole thing in one go, it’s at least better then nothing and doesn’t lead to the type of mistakes you reverted. I’m pretty sure JOSM allows for imports on those levels to.