You can't modify it? Check that 1) you are an administrator on the computer you're using and 2) that your game is installed in a non-protected folder such as C:/Games/ and not C:/programs or similar.
About the event ID numbers, no, you can use any unused number you want.
I can modify the excels, but when I save the changes, a window appears to me saying for example 'mods.csv can have not compatible characteristics with CSV.' And when I save the exel folder, the game doesn't run.