Ah I see!
Yes, its is possible to combine a hybrid tile, although, I am experimenting that hybrid stuff lately for my melee map and not all of the tiles could be imported to another. Using Cityscape tileset would give you majority of it, some tiles from other tileset couldn't be add to the custom tiles. Dirt and other common tiles. Vice versa apply to the Ashenvale. They would be marked with a red cube on the upper left corner of their icon if its not available, you could change them at 'Advanced' and to 'Modify Tileset', check the box for custom tile managing.
I think its a nice touch have it both in a mix.