pretty sure it's not worth it. if you want HD, just get the HD box. otherwise, i don't think there's much a point in bothering with digital conversion, even if you could, unless you absolutely have to in order to get a signal to your TV. simply converting to a digital HDMI signal isn't going to make your video better to look at
a brief search only turned up devices to go the from digital to brown townog tho, not the other way around, since i figure most TVs still have coax input. if the problem is that your TV doesn't i imagine there's gotta be somethin out there but i didn't find it within 5 minutes