as long as you are using the old terragen, 0.9 x, there is an exporter either on the save dialogue or available where all the plugins are found, and it exports .lwo and .raw.... I've tried the .lwo and it worked fine in Imagine3D and what works in Imagine often works in Bryce
Sorry if this is a bit sketchy but I don't have any terragens on this box any more..devoted to the net... ...

 

Once in a while I look around,
I see a sound
and try to write it down
Sometimes they come out very soft
Tinkling light sound
The Sun comes up again