Killmarks?

Killmarks are allowed to customize indeed.

(EDIT: As long as it isn't a rez file, then it is allowed to customize it. Though, keep in mind that if you mess something up, then it is not guaranteed that we can help you with your issue and you may have to reinstall the game. (Like killmarks not appearing in-game at all))

There is even a section at Modhub dedicated to killmarks customizations only.