Why is steven stone in the pewter museum Pokemon soul sliver?

Steven Stone is not in the Pewter Museum in Pokémon Soul Silver. He is actually the Champion of the Hoenn region, appearing primarily in Pokémon Ruby, Sapphire, and Emerald.

There's a possibility you might be thinking of a different character or a different game! Could you clarify which game you're talking about, or which character you're thinking of?