That's not normally possible unless you have a special video card with S-Video out on some of the unused pins of the VGA connector. They do exist, but they require proprietary cables furnished by their manufacturers. Otherwise, converting from a VGA output to NTSC video is going to require special equipment — again, it exists, but you'd most likely be better off just upgrading the video card.
Or get an external "box" like a Focus Enhancements "T-View". It's a little idiot-box that has a pass-through connector that your VGA monitor still gets its relatively unadulterated signal through, and the scan-converter generates a Y/C and a composite signal from, with a few tweaks for centering, size, brightness, and contrast.
If I recall properly, the T-Views we bougth at work were about $150/ea.