Bioshock gamesave
1) Hope you still have old Bioshock saves, thats what I used, maybe it will still work without it, I don't know.
2) I had a auto-save right before the final boss (the same location as the downloaded save) on Hard diffculty.
3) Using that as the base, copy it on ur PC.
4) Download the internet save.
5) in the folder, there is file called MAINSAVE.BSG, copy that from internet save folder, overwrite it over the same file from your PS3 save. DO NOT overwrite anyother save, or change the original folder.
6) Load this slightly altered save back into ur PS3, and the system will be fooled.
I doubt its this easy with other games though.