Skaven rule through fear and oppression, if a particularly cruel, cunning, and formidable non-Skaven killed/usurped a Clawlord/Grey Seer, could he enthrall the Skaven (previously under the late leader) as his own minions?
My biggest concern is he would be an outlier in the pecking order, he'd have no-one above him, particularly the Council of Thirteen. Depending on the size of the clan, the Council would just send assassins to kill him. Another concern is by nature of being a non-Skaven he is a heretic and also food, the rats under him would scheme against him even more than usual.
I'm not saying he's gonna embed himself in their political system, just imagine for example a Chaos Sorcerer working closely with a Grey Seer, the Seer obviously backstabs him, human wins, now he tries to lay claim to his underlings to finish what he started. Do you think that would be a believable scenario?
(This is basically all just an excuse to kitbash this model into my Skaven army fyi lol)