I'll try to answer your question. The 'red, white, yellow' inputs on your tv are 'composite' inputs (not digital, i think). the red/white is usually for the audio portion - maybe an audio output signal. anyway, these give you the lowest signal quality. The S-video input you mention will give you better picture quality.
You cannot use the comcast DVR to add other inputs (gamecube, vcr, etc.) to your TV.
You can buy an audio/video switch box to do this. they are available for composite, svideo, component, etc. inputs. I did this with an S-video switcher from Sony - only about $40. it has room for 4 different sources - cable, vcr, xbox, gamecube, etc - for both the audio and video portion of the signal. It 'automatically' senses the active signal and outputs this to the single output - to TV. worked nicely.
i just swapped this out to a component video switcher. this uses the 'red, green, blue' video inputs - which gives an even better signal than svideo.
hope this makes sense. i would recommend looking for an svideo switcher - if all your sources can output s-video.