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You want to add a fat morph to a clothing mesh? (So that sims actually look fatter when they become fat.) I'm not sure if it's possible to have just the fatmorph and I don't see the reason if you have the regular size already. Then again, I know there are maternity outfit defaults, so I guess it's possible to have plus-size-only outfits.
Most of the clothing meshes shipped with the game has the fat morph included so when you make a clothing recolor it fits the fatmorph too. You don't need any special texture for the fat morph. (It's a property of the 3D-system used by the game.)