There are PSX to USB adapors at radioshack for $10. Depending on how the PS1/2 emulator works in PS3, you might be able to use that and hook it up via USB.
Otherwise there a few possibilities for DDR on Ps3: organized tournaments, music and step sharing, downloadable backgrounds, avatars, etc
DDR isn't really my type of game either but I'm not going to knock it. Would you want people say "Who cares?" about your favorite game?