It is possible to reduce the polys of a super high-poly model way down (to 2K or lower), but it takes a little work, and of course you loose an enormous amount of detail. On the other hand, you can avoid the licensing issue entirely if you use only the reduced mesh, draw your own textures, and U/V map the model yourself, because you are not using any part of the copyrighted material in your program, and the reduced mesh is quite different than the original high-poly mesh.
I've never actually done this for Poser, but I have for Daz Studio, which is basically the same thing as Poser.
What you need for this method:
1) A way to convert .obj into .wrl (I use Autodesk 3DS Max - you'll need the proper importers and exporters for your modeling program)
2) VIZup Free Edition, version 1.8 or earlier (later versions require a paid registration to export)
3) A way to import .wrl into your modelling program for editing.
What you do:
1) Start by exporting only the head of your Poser model into .obj format. I usually remove the eyes and add them back later after reducing the head's polys.
2) Convert the .obj into .wrl (discard any textures, since you are not using them anyway)
3) Load the head .wrl into VIZup, and reduce the polys to as low as you can get them while still being able to distinguish the nose, eyes, and ears (Usually can't get any lower than around 800-900 polys). Don't worry about a few gaps in the mesh - you can weld these shut later.
4) Save the new low-poly head to .wrl format.
5) Import the head into your modelling program.
6) All the vertices will be disjointed and there will be gaps in the mesh. This is where the work comes in - you will need to meticulously weld together vertices that are close to eachother to fill in the gaps. Avoid the temptation to divide or add polys unless absolutely necessary - poly count adds up quickly. This usually takes me about 6-8 hours to do for a head to make it look nice.
7) Add the eyes if you removed them (may require adding a few extra polys around the eye sockets first).
Repeat this process for the rest of the body. I usually remove the fingers and toes, since they disappear during reduction anyway, and add them back later (as I did with the eyes). The reason I do the head and body seperately, is because the body can usually be reduced further than the head (around 400 - 500 polys at the minimum).
After combining the head and body and welding them together, you can texture them, set up a skeletal heirarchy, and animate. The entire process takes several days, but there is a benefit to creating models this way - there is a more natral non-symetric feel to the model which is very difficult to produce from scratch (models designed from scratch are often "too perfect", which gives them an strangely uncanny appearance if you try to texture them with actual photographs). I find that low-poly models produced with this method are much easier to texture, and the imperfections are much more believable.