Err, I think they should stay as separate VOLs. I'm not really sure what the copyright or licencing status is on the Green World stuff. Plus, I'm just in favour of keeping the original stuff separate from extension stuff.
@Fire Plague, the maps choose which tilesets they reference. You play a Green World map to see the Green World tileset. Not sure how well things would match up if you just replaced the original tilesets with the Green World ones. The tiles would likely be out of order. I suppose you could always try it and find out.