You already CAN take tiles from other chipsets and put them into another chipset.. You just have to overwrite other tiles in the process. The big tree in the Basis tileset is 2 tiles by 2 tiles.. that means, if you want that in another tileset, you have to overwrite 4 tiles. It should be in the upper layer side... it's easy to tell which layer is which.

To simplify, pretend that each tileset only had 10 tiles. If you wanted to put a new tile in, you'd overwrite some other tile, AS LONG AS THE SIZE OF THE CHIPSET REMAINS CONSTANT. No matter what, our theoretical tileset would only hold 10 tiles... it doesn't matter what those tiles are.