I've only used TS enough to realize that I never want to use it again (

), so bear with me here and overlook my stupidest questions.
Why don't you export Terragen's Height Map to Truespace, create a plane with several thousand polygons and apply the image from Terragen as a displacement map? That would give you Terragen's terrain in Truespace (admittedly with a lower polycount and, as such, with less detail. However, that is not necessarily a bad thing, because Terragen likes to think in terms of
millions of polys, which is not ideal for in-game use.)
That's what I do with 3dsMax, and it works rather well...