Thank you Dune! I've just been looking at what files Poser & Terragen could import or export. It looks like the only file types they have in common are .OBJ and .LWO. TG also says it can import ".OBJ (and .LWO) as populations"... I don't know what that means, but I'll have to check it out.
In the past I have saved Poser Figures (.CR2) as .OBJ and then imported them into another program, but that was not satisfactory; A .CR2 Figure as an .OBJ is just a 3D wireframe with no texture, morphs or articulatable skeleton. Nevertheless, I may try it (maybe "as a population") in Terragen.
I had hoped that a TG landscape was basically a wireframe that I could import into Poser as an .OBJ, and hopefully with its texture. But you pointed out that would be hard (and no doubt too difficult for me). However, there remains the possibility that I could texture the .OBJ in Poser. (If it's a flat-to-low-hills desert terrain, I have the textures for that, anyway).
However, Dune, it looks to me like my best bet is to create a landscape, sky & atmosphere in TG, save it as a .TIFF, and then simply use the TIFF as a background picture or on a Prop in Poser.
Thanks again. I'll be trying out these various things.