I believe ArchiTerra3 from Cigraph will do what you want, see here for further info -
http://www.cigraph.it/cigraph/pagetrans.do?action=current_product&prodotti_id=24&lang=en
This thread discusses it further -
http://archicad-talk.graphisoft.com/viewtopic.php?p=98536&highlight=architerra#98536
I've not used it myself, but I believe you may need to have the add-on/plug-in loaded on every machine you use, if you want to edit the terrain further. (I could be wrong!)
It may be worth reading up on the positive and negative aspects of using add-ons/plug-ins before buying. There are plenty of posts in these forums about the subject!
Perhaps someone who actually uses it may be able to comment further.
Hope that's helpful!