Never seen this mod in actual use, but from looking at it, I would think creating something similar that works for all actors (including the mod-added) would be doable. What I would do was to start with an empty array. Whenever an actor was killed, its base object id is looked up in the array. If the base object id doesn't exist, it is added as a new entry in the array with kill count 1. If it already exists, the kill count is incremented.
When displaying the kill stats, it is a matter of looping through the array, extracting the name and kill number and printing the info. Grouping could be a bit of trouble, but there's some general functions to do some grouping (IsCreature, GetCreatureType, GetClass, etc.)
There's of course a bit more to it, but it doesn't seem too difficult.