[old_tv_nut]

It's generally possible to do a mod to add composite input to a color set (and in some cases S-video), and generally not feasible to add RGB input. The basic idea is to switch the composite in, in place of the video detector output, but you may need to tweak the design to match the detector peak-to-peak and DC bias values.

Not recommended for a hot-chassis set, even with an isolation transformer.

I think you may find some threads here on AK about doing this.