What do you want to export? Terrain? As a heightfield or geometry? Environment or skybox/spherical panorama for HDRI/IBL? Camera and lighting? There are numerous possibilities, but also some limitations. If your intention is to model something in Terragen then export it to Maya and hope to get similar - or even better - render quality vs. rendering it natively in Terragen, that is probably not the best approach. If you want to export assets for use as hold-outs, shadow-catchers, occlusion, etc. that can work better.
- Oshyan