You can set the "Conversation Match %" from the bot's learning page. The bot will only use a response if the response correctness factor is above this %. The default for conversations is 50%, for discussions (chat rooms) it is 90%.
If the bot does not have a good response it will use its default response. In its default response you could have it response like,
"Sorry, I do not know a response for this, please email firstname.lastname@example.org, or chat with a live agent"
I don't think you would want the bot to send an email and hang waiting for a response, as the user expects immediate responses from the bot, waiting for a human would take too long, as well a conversation may have a lot of missed responses so it would be sending a lot of email (think of a user talking a different language).
You can add a bot to your website in two way,
Bot - The user chats with a bot directly with an avatar and tts
Live Chat - We also support live chat, and you can link a bot to a live chat channel. Users are queued for live operators, but can choose to talk with the bot if no operator is available.
What you want is Live Chat. Live chat supports an operator mode "spy mode" that lets the live chat operators monitor the bot chats. They can also take over a conversation from a bot using "accept" or "private" commands. This lets a single operator monitor many conversations and provide a sort of tier 2 level of support.
We can provide a customized solution for you as part of our Bot Libre Platinum service, or our Paphus Live Chat Enterprise plan.