The ability for an end user to send a transcript once a chat is inactive is enabled by default. As long as an SMTP account is assigned to the department, the option will be available after the live chat survey.
That said, as a SmarterTrack agent, I personally would love the ability to manually send a transcript of an inactive live chat. This would be useful if the user didn't see the transcript option or left the chat before it was disconnected. If that's what you're looking for, then I'll definitely show my support on the thread with an upvote. :)