Allowed to mod texture files on client side?

[ThinkGreen]

Hell0,
i read a few threads and i think i know the answer but just to make sure, i wanted to ask here for a confirmation.

So, from what i understand, we cannot change the textures (Exemple: texture files for characters) through addons, but we can actually mod them, is that right? What i mean, is, is it similar to other Beth games such as Fallout and Skyrim, modding a texture file and adding the folders with the correct hierarchy in the data folder?

Thanks!