Can you turn a hero chao into a dark chao

Yes. After spending 15 hours in the garden (collectively) your chao will reincarnate (if treated well). It will return to egg form and rehatch into a child chao, so that you can start over. It will retain 10% of its stats from its previous life.
Also feed it the fruits from the dark garden and give it at least a few dark fruits from the market even before or after it is/turns back to a child chao.