Oh I have done this before. The files you need to edit are the trees.gp and the stone list or the tree list. Which ever one. All of which are found in the All.gsc file.
Then if you wanted the trees from Divided nation you would extract the trees.gp from divided nation and make a gp or just use the Divided nations tree.gp.
Then you would edit the stone list. This list is kinda tricky as it fooled me, but I have edited the trees and added more. There should be tutorial on this as it would be very useful! Just remember as with all lst files there is almost always a number counting how many strings there are.
Anyways I hope this helps and if you need more info just ask....
Move this to the Ac forums! hehe