While searching for more information I found this https://blenderlex.wordpress.com/2015/03/12/installing-insane-bump-for-gimp-windows/ which led me to these GIMP plugins:
normalmap - http://registry.gimp.org/node/69 (haven't tried this)
Insane Bump (Python) - http://registry.gimp.org/node/28117 (tried it but it needs normalmap?)
Insane Bump (C) - http://registry.gimp.org/node/28638 (with the DLL in the same folder this works)
My first play with it (the C version) worked nicely. Here's thumbs of the maps created from one of my photos:
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Here's a Poser 9 render using these maps together:
Here's the shader I used:
And here's what the render was like with just the texture:
I did some checks and confirmed that the normal maps created by the plugin are correctly oriented for Poser. They're tangent space, with green being down-up (0 is down, 1 is up) and red left-right (0 is left, 1 is right) when viewing the normal map in GIMP. That makes sense to me even if it doesn't to you! ;o)