Thanks to Blaster_nl
I finally managed to add original trees on my custom map.
The procedure is to export the original object and import it with GIMS on the scene.
The object has to be placed like the other map objects, but it has to be named like the original file, and the cloned ones have to be named the same.
I have to test the collisions, but I think that the export process for them is managed by exporting the OBN and WPL files only.