After the Chatbot could not answer the customers request in a conversation. i want to give an option "talk to a real person". Is it possible that a admin can take over a conversation?

You can take a look at zendesk integration

And if you are using a WhatsApp channel (or maybe other ones too), you can send the user a whatsapp link with your Agents phone number: https://faq.whatsapp.com/5913398998672934