There was one device for the old, fat Playstation. The earlier models hat a serial and parallel port for a possible hardwarte extension, which never came. There was a device called Power Replay and one called Goldfinger. They work like an Action Replay back in SNES days. So you could enter cheat codes and it would let you play import games. Since it plugs into the back of the system you wouldn't have to do any modifications on the system.
As for the PsOne, I'm pretty sure you're stuck with a modchip though
