Welcome, @paulrudy! I don’t see an existing Zulip importer. Since Zulip is more of a chat than a forum, I guess no one has wanted to do that (or if they did, they didn’t bother to contribute their code to Discourse).
If you’re familiar with Ruby, or programming in general, you can take an existing importer and modify it to work with your Zulip database. I don’t see any importers that read a postgres database, but since it’s what Discourse uses, it shouldn’t be too hard to figure out.
If you don’t have the time to do that, but do have a budget you can seek help in Marketplace or contact me via my migration quote form .