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I read on one of the NW3 README files that one could place certain files to change player's hands (in appearance). I was wondering if any of those existed because I can't seem to find any on the interweb,

They are in the official release of NW3 as far as I know.
Feralidragon put a whole documentation into the download.

NW3 does support that, meaning that theoretically you can indeed download or create your own custom hand textures.
However, I think all the existing hand textures exist in NW3 alone and no one ever did custom ones, although NW3 itself has quite some of them, especially for each species (human, nali, skaarj).

The documentation corresponding to how to do this resides in the INI_NWConfig.pdf help file, namely the NWCoreVIII.NWHandsInfo section.
Although I don't say there, custom skins must be in their own texture files (as textures), and then you can just reference the package and textures in the .ini file.