WC3 doesn't use .mpqs anymore for it's own data, only for maps, and you shouldn't try to modify the CASC archive for a variety of reasons.
If you want to overlay additional content on top of the engine, such as for a full-conversion mod, then starting from patch 1.31 you'll be able to use a directory with a `.w3mod` suffix in the game folder to "overlay" data onto the engine.
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