I don't have to, any more than Illwinter had to include Arthur, Mordred and Merlin for the nation of Man. It has Knights of the Stone, i.e. of the Round Table - but there's no "Arthur the Great King", no "Merlin the Demonbred Sorcerer", or even a sword in the stone. There's nothing like the scabbard that heals the wounds of its user, and I doubt Stone Sword is Excalibur.
I could take a totally different interpretation of Kullervo, like Tolkien's Turin Blacksword, and use that in my mod. Sword Companion, medium infantry with metal armor and enchanted swords
I could even make it a new Nature/Death item the mod made available. Constr 6 or even 8, armor-negating dagger with special ability of "curse luck" on hit - cancels Luck, makes more suspectible to afflictions and sucks in general. If advanced item modding commands/ "copyitem" were possible, I could make it cursed and/or make it autocurse the wielder.
I won't add Kullervo just to get him added. I don't want too many units or commanders that aren't used, and I don't want to add units that I can't make feel right even to myself.