When and how do I assign UV maps in Poser 13? Is it done in the material room? Cheers

If you mean something that isn't UV mapped...I think you have map them in other applications like UV Mapper or Blender.

uv domain mapping (vertices and polygons) usually comes, in the .obj file as one set with the xyz domain mapping. Same holds for uv domain mapping.

There is, in Poser, currently no method to combine or swap out uv maps. If you have a separate mapping for the uv domain, but with consistent, or at least mapped, order of the polygons as in the xyz domain mapping you could write a small script to combine them, either in memory or in a .obj file. 

If you want to pursue this your main tool there would be the below method in the geometry type:

AddGeneralMesh(...)

Add a general mesh to existing geometry.  Arguments are numerical python arrays specifying polygons, sets, and vertices, as well as optionally texture-polygons, texture-sets, and texture-vertices.  (See Numerical Python documentation for more details).
<NoneType> AddGeneralMesh(<IntType nx2 Numeric.Array> polygons, <IntType nx1 Numeric.Array> sets, <FloatType nx3 Numeric.Array> vertices,
 {<IntType nx2 Numeric.Array> texPolygons, <IntType nx1 Numeric.Array> texSets, <FloatType nx3 Numeric.Array> texVertices})

polygons, sets and vertices can be obtained from the xyz  geometry using the proper methods. The optional input parameters texPolygons, texSets, texVertices would have to come from your custom uv definition. This could be a normal .obj based input. A polgon (facet) definition in a .obj requires vertex data. You can do with just 4 vertices repeated for every polygon.