The Discourse integration on Zapier gives the options to either create a new topic, or to add a post to an existing topic. It does not have an option to first create a topic and then add posts to that topic. I think the functionality you are looking for could be achieved with Zapier by using their Webhook action. There are some details about setting that up in this topic: How to make requests to the Discourse API with Zapier.
What application are you using to trigger the Zap?