In Pokemon diamond can you catch a level 100 Pikachu?

you can get one, but you can't capture it. Just capture a pikachu, train it to level 100, and avoid using thunderstones on it.

there is a code for lv99 and level it up one

Yes you can catch a level 100 pikachu only if you migrate one from leafgreen or firered or ruby or sapphire or emerald.