You can always change materials of your ecosystem objects. In the material tab of the world browser, look for the material you want to change, double clic on it, and make your changes. As for saving to .veg, not possible with an ecosystem object, only when it's a "real" tree. If you choose "save object", it will be saved as vob, regular polygon object, but if you choose .veg, then it's another SolidGrowth plant, with all the advantages of SG.