good morning
I would like to know if there is means to import trees and rocks in sapien as on halo CE or if it is necessary to create them on 3dsmax.

thank you

You can make trees in CE and use the gbx model upgrader and go through that whole process but the problem is that it will be render only. There is no way to compile or convert CG "collidable geometry" into the halo 2 format. In my opinion this is just so we can't make custom weapons or sapien placable items.
I read that they set up halo 2 vista's map pack up with a new "innovative" solution to scenery placement, And the solution is "Instanced geometry". I say it's lazyness because they did not care if we have a porper tool, they wanted the money. Instanced geometry is just placing an object in 3ds max as a seperate object and you just put a "%" before the object's name. Not the material name the object's name.
What that does is compile the instanced geometry "a.k.a. a tree or rock" and the bsp seperately so in essence it renders seperately. I may be wrong but to me it's like the scenery is on the other side of a portal, if you know what a portal is.

So in short. look up instanced geometry. lol