Like all of you, I was looking for a way to enable the Japan-exclusive shadow Pokémon trainers into my PAL/NTSC-U games.
Thanks to javier_himura's efforts 4 years ago, I was able to create WiiRD/Gecko and Action Replay cheat codes that will inject the e-card data into Pokémon Colosseum.
It is not possible to include this feature into a save file editor because the shadow trainer data is located at 1 specific location and will overwrite the data of any previous trainer.
You can find the cheat codes and instructions at the following links: WiiRD/Gecko cheat codes for NTSC-U games WiiRD/Gecko cheat codes for PAL games
Action Replay cheat codes for NTSC-U games Action Replay cheat codes for PAL games