overrides and patch HAKs can not reskin HAKs associated with a module.
For what it is worth, the city override that I used (in that Llyra patch hak) were put together by _six. So you can get the originals there if you want to go to the source.
what Sen probably did is renamed the textures and made them darker. if you extracted his textures, you would have the list of file names and a visual reference for which is which. This would help you assemble a new hak on top of which has textures to override Sen's. This would includes six's textures renamed to match sen's textures. You will also need to make sure that the textures in your HAK include DDS versions s that the override takes, and both TGA and DDS can be useful for some graphics cards. I probably have DDS versions in the patch HAK contents if _six didn't do this.