In order to configure this outcome, first you'll need to utilize a custom implementation, here's an example [1].
You can set the action "input.unknown" within Dialogflow is used to deliver the converstaion to a live agent, which then triggers the default fallback intent and the associated action is then added to the response (handoff to an agent).
You can also integrate with a third-party tool to handle the handoff, as mentioned in more detail here [2].
There is also a pre-general available offering in which a Dialogflow agent can be designed so that certain matched intents then triggers the Dialogflow agent to hand off the conversation to a human agent. There are 2 options that help to achieve this [3].