Brook cannot have another Devil Fruit, unless Oda-sensei's hyper-creative imagination provides him a loophole.

What if Brook ate another Devil Fruit?

This caused fans to think he meant if you eat the second Devil Fruit it will replace the first fruit but in Enies Lobby Arc, Lucci and Blueno said if you eat the second fruit after eating the first one, your body will explode and you will die.

Does Brook eat a Devil Fruit?

Devil Fruit

Brook's ability to separate his soul from his body. While he was still alive, Brook ate the Yomi Yomi no Mi, a Paramecia-type Devil Fruit that allowed his soul to return to the world of the living and reinhabit his body after he died.