As for why he refined these gourds together, it was because they were of little use to Zhou Chen.
As for returning them to their origin and smelting them into an Innate Supreme Spiritual Treasure as Senior Brother Yuanshi suggested, leaving aside whether this method was even feasible, simply gathering all the gourds was quite troublesome.
The Immortal-Slaying Gourd was in the hands of Lu Ya, the tenth Golden Crow, and another gourd was in the hands of Haotian. These two gourds would be somewhat easier to obtain and could be traded for using Spiritual Treasures or other items.
Chapter 89