Just a reminder that providing specifics on, sharing links to, or naming websites where ROMs can be accessed is against the rules. If your post has any of this information it will be removed.
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Download Advance Series, where you can replace Pokemon Sprite Icons and Positions. Then you can change its data for movesets etc using Pokemon Game Editor. You can find all 256x64 ds styled sprites in chaos rush's thread. Just google search it. If you want to completely add and not replace, you can look up a tutorial on expanding the pokedex. You'll have to make and properly format your own fakemon, though.