Here:
http://respam.terratalk.de/ReSPaM-Install.exeyou can find a program which does the job for you, if you just want to build panoramas out of BMPs (that's the only filetype that can be used), not HDRIs.
In your favorite 3D program set up your camera with a FOV of 90° and take six pictures: up, down, back, front, left and right, all exactly 90°.
The program is in german, so for those who don't know:
"Himmel" means sky, "Links" means left, "Rechts" means right, "Vorne" means forward, "Boden" means bottom and "Hinten" means back.
"Neu anfangen" means start from scratch and "Panorama machen" means make panorama.
"...drehen" means turn.
Believe me you won't see any seams. I tried it with the old Terragen-version and it worked perfect.
In the end you will have a large file with a strange ending. Just rename it to xxx.BMP and this will be your pano.
In an app like 3ds Max map this image spherically onto a sphere.