Not all that familliar with Photoshop but you should be able to render the Lost Realms scene seperately.  Export the render  as a PSD file, bring it into Photoshop, apply the clamshell, save the whole as a jpeg and import it into Poser as a background.  You should then be able to put the figure in possition with the clamshell.  I think you could also make the clamshell a second layer.  That would allow you to easilly move it if you find that it isn't in the right place.  
     The hard part will be making sure that the clamshell is an appropriate size and distance into the setting to allow you to pose the figure where you want it in the overall scene.