Using content from another game is not allowed, however, if said content is already in the game, then Bethesda does not mind. Because there is no Trog texture in NV, it would be ILLEGAL to use the texture in NV. If all the assets were there in NV but just unused like a lot of other assets, then yes, it would be fine because the files came packaged with the game. IE: 90% of Mothership Zeta models are included with NV, though 89.99% of them are not even in the actual game.