Ok, thanks guys...
I've finally made it! I had to use Dolphin, yep... Thanks Fishaman for the clarification by the way.
I followed AlanWeird's steps, although let me explain it in steps for those whose of us who had a bit of a hard time understanding the procedure:
What you need:
1) The edited Japanese bonus disk .iso file
2) Giftpia iso (only in Japanese)
3) Dolphin in your computer
4) Memory Card Manager in your Homebrew channel
5) A free spot in your Colosseum team
Procedure:
1) With your memory card inserted in slot A in your Wii, backup your Colosseum save in your USB drive using Memory Card Manager.
2) Turn your Wii off and connect your USB drive in your computer.
3) Open Giftpia with Dolphin and let it run for a bit until you see a yellow warning on the upper part saying something like 'a memory card has been created' somewhere (usually C:\Users\user\Documents\Dolphin Emulator\GC\MemoryCardA.JAP).
4) Close the game display, but not Dolphin itself, and go to Tools > Memcard Manager (GC).
5) On the left side, click on Browse and look for the Japanese memory card (step 3). Select it.
6) Now click on Import GCI and look for the backup saved in your USB drive. Select it.
7) You should see two rows in the left side, one for Giftpia and another one for Colosseum.
8) Close the window and now load the edited Japanese bonus disk on Dolphin.
9) You should now be able to receive Celebi. Once you do, your Japanese memory card will be automatically saved. Close the game display.
10) Go back to Tools > Memcard Manager (GC) and look for the Japanese memery card (step 3).
11) Highlight the Colosseum save and click on Export GCI.
12) Now save it in the same directory where your backup is saved in your USB, but do NOT replace it! Just name it something else, for example, instead of the last _00, replace it with _01.
13) Connect your USB drive to your Wii, open Memory Card Manager in your Homebrew Channel and Restore the new save (the one that ends with _01).
14) You now have a physical copy of a Colosseum save with an original Japanese Celebi in your team. You can even use it in your Colosseum original game or trade it to your GBA games.
Congratulations!
And thank you all for your help.
Cheers and good luck!