I’ve been playing with Zapier a bit and am able to use it to add a new topic to a category in my Discourse site. So far it’s performing as expected, except for one thing.
There doesn’t seem to be a way to both create a new topic and add replies to an existing topic from Zapier. If I send a 2nd post with the same topic as a prior one then I get an error instead of it being added to the existing topic as a new reply.
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?