The only thing you'll find of any shape, form, or fashion, that will allow PS1 games on a Dreamcast is "Bleemcast", which doesn't allow all games to work, but rather one or two titles per Bleemcast program. They have made GT2 (?) and Metal Gear Solid (?) versions that I'm familiar with, and possibly one or two others by now.
They can be purchased for about $5 to $10 per title at most stores that sell Dreamcast and PS1 games together, such as Electronics Boutique, Game Stop, etc. You can also find them for sale online at
http://www.bleem.com although for some reason I can't connect to it right now, so they may have gone under...