You can't really merge two tilesets to make one big tileset. They have to stay the same size (like the face poses, you can only have 16 of those and they must be a certain size.) You COULD take the best tiles and put them on your own tileset (overwriting other tiles in the process) but it won't read any farther than the size of a standard tileset.

Each tile: 16x16 pixels
Total tileset size: 480x256 pixels

Which means its 30 tiles by 16 tiles... 480 total tiles. You can mess around with that...