1.)  In P5, P6, and P7 the Poser Default Ground Plane PoserSurface node is at: 

Material Room > Advanced > Object > Prop > GROUND.  

2.)  You texture a PoserSurface node the same way for any Figure, or Object, in your scene:

Create an Image_Map node.  Load/Browse to a Texture, plug it into a White Diffuse Color node, and Set the Diffuse Value to 1.

3.) FIle > Import Poser scene.document, will merge two scenes.  Otherwise save whatever it is you want to Export/Import to a Library.