"...I am just asking if anyone has added an extra input and how they did it..."
Well, as noted above we (Bottlehead) have done it, and that is how we did it - replacing the volume control with a version of the Submissive.
There are also many stand-alone selector boxes, often with video switching capability as well as audio. Customers have used these going back to our second product, the Foreplay preamp from 1998. They are quite inexpensive.
Hopefully someone has done the kind of mod you are thinking of, and they will post.