The only possible workaround is to open the file where is your ArchiTerra terrain and save anything as object (BINARY will make anything faster but it depends on your 2D representation need).
Then use this object (also like module if you want) inside the other plan file.
If you need to change the terrain, just come back to the terrain project, make the modification, and save again anything as object overwriting the previous one, the project file (I mean the file where you inserted the terrain object) will be immediately updated.
Friendly
Fabrizio
Fabrizio Diodati
Graphisoft Italy Srl | Via Rossignago 2/A Spinea Venezia 30038 Italy