I used ($$) 3DOC to convert either the ~2 MB XPS or ~4 MB Generic_Item.mesh.ascii files to ~2.5 MB OBJ+MTL.
Three ''mountain'' and two ''plant'' objects were supplied as textured ''flats''. Poser does not automatically (*) recognise textures'' embedded ''transparency'', I had to make these myself. The masking rules are, for ''Super-ambient'' glow, black is dark, white a-light. For ''Transparency'', black is transparent / hidden, white is solid / visible.
There are probably several better ways to do this, YMMV.
Each part of the mountain back-drop has a white ''sky'' above its vari-coloured slopes. I used free Irfan View''s paint tools with a low threshold to flood-fill the white ''sky'' to black. I then used the image tools to mostly change the landscape colours to white, finishing off with paint tools and a white brush. I saved file with an N or T name-suffix for ''Negative'' or ''Transparency''.
The two plants were too intricate to be done that way. I used other Irfan View image tools to reduce their ''foliage shades on white'' colour depth to ''Black plant on White'', then inverted the colours to ''White plant on black''.
The scene imported ''very small''. I had to scale x10, to ~1000% original, ''translate'' by ''X:0, Y:135, Z:-500'' to bring meadow to centre of Poser scene. Took some experimenting with camera position and ''focal length'' (~15mm) to mostly align the scene back-drop''s components. Given the sheer extent of scene, I also had to hide Poser''s own ground-plane / back-drop and max-out the camera''s ''yon'' limit.
The 18 materials'' diffuse textures had auto-loaded per MTL. Adding their ''transparency'' maps converted the five ''painted flats'' to plausible scenery. I also used the diffuse textures as bump-maps, for a bit more ''3D''. I thought I''d enabled enough ''displacement'' to turn the flat meadow into low grass, but seems not...
Several quick local ''superfly'' renders showed scene needed more light. And yet more light. And then just a bit less than that...
Sent ''superfly, 96 pixel-sample'' job to my networked ''Box'', which ran for an hour...
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Note: If you do not have access to ($$) 3DOC, there is a route via DA''s Mariokart64''s free ''XPS to PMX'' utility, then DA''s free PMX Editor''s OBJ+MTL export options.
*) Can be *set* deep, deep within the arcane zoo of ''Material Room'' options, at least for PNG ''Alpha Channel'', but is not automatic, and I''ve not tried with DDS...
Wow, thank you kindly Nik 
! This looks like an impressive scene 
!