I don't think you can really "add" or "make" any thing new to the Spore game. If you mean "Crack' then follow Daft's plan.
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A program called Textmod can be used to skin Spore, but will not change gameplay. It can find and replace various textures, and works with a multitude of games. Here are instructions on how to use it. The instructions are for a different game, bu the same principals apply.