Well, you get your gamesave onto your PC with the sharkport....get MK's hybrid program, mod your cars, write the garage, and then send the save back to your PS2 memcard
Go there, look towards the top of the page, and find downloads. Then go to the GT3 section, go to hybrid programs, and find GTstat5 or something like that, it should say 5, and then click and it will begin to DL.