Rasa has documentation covering how to allow the bot to escalate to a human agent, along with sample code for connecting to ServiceNow. This could be used as a starting point when building your Zammad integration:
I think it is possible to build a solution using Rasa and Zammad API + Webhooks. But it seems pretty complicated, you will probably need a developer at your company to build and maintain the integration.
Zammad does includes a web chat channel though. If the amount of chats worked by your team is small, it might be worth considering using it as-is, instead of integrating a chatbot: