Alex,

Heres a word for you about spt, you cannot use spt files with 3dgs unless you have a coder who can write the intergration for you. spt files are native to speedtree cad which is a stand alone application that you use to modify the tree assets you get with speedtree. There are exporters for Max and Maya for leaf meshes and for forest creation but thats really of no use to you unless your engine can support speedtree. So in short I cant tell you how its done, but I can tell you that you need a skilled coder to do it.
Now on a personal note there are some great trees in the speedtree package but I think that there are artists who could create better trees and also make them sway and so forth. Getting speedtree for making huge forests would be the only reason I would ever get it. Other than that I dont see the need because everything that it does can be done with the engine now. Of course you would have to write code for that but I think most game engines could handle what speedtree does. GS out.