is it possible to group several objects to be used in the population?
Thanks a lot!
You can do this out of Terragen if you have some 3D software like Blender or Poseray. Keep in mind that your group will come in exactly as positioned in your group and after grouping you should set an origin. Then also set rotation limits and scale limits once imported. (Group will vary by group, not within the group)
But grouping usually means that some parts/objects are away from the center of that group and in hilly terrain the parts/objects on the outside of the group may well float. They won't sit on ground.
This capability is not currently supported in Terragen, but we do intend to add it in the future.