It can be done but it's a little fiddly. There is no"spherical projection" as yet in Terragen
Firstly make a planet with a texture you like, then delete the planet and replace it with a Terragen sphere the same size and location as your planet. In the spheres node place a (-) in front of your sphere's radius. (So if your using the default planet it's radius goes from 6.378e+006 to -6.378e+006 )
Now delete the atmosphere, you will have to add this separately in your game.
Now place a camera at the centre of the sphere and set it's FOV to 90 degrees. Set your render dimensions to say 1000x1000.
Place a light source in the centre of the sphere and adjust it's strength and max distance to illuminate the inside of your sphere. Switch OFF cast shadows, glow in atmosphere and specular highlights.
Now you are ready to render. Point the camera straight up, straight down, forward, backward, left and right. Do this in the camera dialog NOT by moving the camera manually. You could animate this last process or do it as separate renders
You should now have six tiles ready for UV mapping back onto the sphere in your game.
As I said, this is a little fiddly but it will work.
Hope this helps
Richard