From the TG2 FAQ:
http://www.planetside.co.uk/terragen/tgd/tg2faq.shtml
Is it possible to export an entire scene to another 3D app?
This is currently undecided. You will certainly be able to export elements of scenes, including highly detailed geometry. Exporting highly complex surface or atmosphere shaders would be a more difficult proposition. We are very interested in making the integration of TG2 and more general purpose 3D apps as straightforward as possible.
http://www.planetside.co.uk/terragen/tgd/tg2faq.shtml
Is it possible to export an entire scene to another 3D app?
This is currently undecided. You will certainly be able to export elements of scenes, including highly detailed geometry. Exporting highly complex surface or atmosphere shaders would be a more difficult proposition. We are very interested in making the integration of TG2 and more general purpose 3D apps as straightforward as possible.