The UV textures for the humans in Poser appear to "seam" their UV maps so that it is impossible to get good texturing of feet and soles to fairly replicate unpigmented skin for black characters. What would be the best way to solve this? Could I create my own UV maps in an external application and bring them in properly to Poser 6? Any thoughts or suggestions on this would be appreciated.

If you mean the figures included with P6, I honestly don't know since I don't have it yet. As for other (eg DAZ) models, altering the skin colour for the palms and soles is relatively easy. Well, as easy as texturing in general, that is. Yes, you can remap the figures but if you're making textures to redistribute, I really don't recommend it. It's another step for the end users, which will often put them off using your products.

It depends. If you are using them for your own use only then remapping the UV is certainly an option. If you plan to distribute them then it's not such a good option. In that case your only choice is to do lots of work with blending. My advice would be to start the pigment actually slightly above the seam (only by 1/2 doz pixils or so) this way it would be hardly noticable (that you started a little higher) but it will be slightly easier to blend.