You could, but it would be more difficult (though easily within the ability of the NSA).
To contribute to the Windows kernel you'd have to get someone hired by Microsoft, who presumably check their employment history (maybe), they have to actually go and work for Microsoft, etc. Obviously none of that is impossible but it's also obviously much harder than sending a patch to a mailing list.
Or you find someone who already works there who's having an affair and blackmail them into working for you. It's no different to how spies have operated for centuries.
To contribute to the Windows kernel you'd have to get someone hired by Microsoft, who presumably check their employment history (maybe), they have to actually go and work for Microsoft, etc. Obviously none of that is impossible but it's also obviously much harder than sending a patch to a mailing list.