I've gone the other way - computer VGA out, converted to TV RCA in signal. Box cost >$100 and the result was very iffy on text, OK on images.
Put simply, the people that made TVs never talked to the people that made computer monitors - apples and pears. I expect a conversion box exists, but I suspect the result may be less than acceptable?
Looks like some "boxes" exist - this will be US NTSC? :-
http://cgi.ebay.com/GE-TV-to-PC-Moni...QQcmdZViewItem
Here is something in Australia - whizz-bang, but $358!! Probably get a small colour TV for less!
http://micronica.com.au/catalog/atv3000/