I don't have B6, but B5 and earlier have wonderful terrain editing tools. I know that Bryce terrains can be exported to a variety of formats, but I don't know if that includes .ter or not - I suspect not, however.
MojoWorld *does* export terrains as .ter and it also imports .pgm, which is one of the formats that Bryce exports.
so, it's not too unreasonable to export from Bryce as .pgm, import into MojoWorld and then export from MojoWorld as .ter... not to mention that MojoWorld can make some pretty cool terrains on its own and can directly export to .ter - I've been playing with a bunch of different .ter files I've made w/ MojoWorld in T2 with great results.
if anyone is interested in getting the 'calyxa special' deal on MojoWorld, please contact me privately.