Just as you can add pictures, flash and text to the screen, if you select "Source" (Right click your screen, go to Area --> Source) you can add an external input source.
You can then place and position this on your screen design as with any other element.
From the source options, source list, simply select the same "source" that you would manually change the TV to, to play content from that input source. Bear in mind that this is local to that screen/TV and so you must have an external source plugged into the particular device.