Yes you can. Go to the object editor, then create a new doodad based on your tree. There are 2 fields named "Maximum pitch angle" and "maximum roll angle". I'm not sure which one of those you need to modify (I think roll angle), but when you enter a negative value (press shift - make sure negative real values are allowed in WE), every doodad of this type you place on the map will be rolled for a certain angle. For instance: changing the value into -3.14 (pi) will turn the doodad upside down.