Rather then to go through blender . it might cause at times some issues when Imorting a Fbx that would not apear in Poser you just Import your model into Poser ... is the aspect ok then export the whole as Obj in Poser this will join all Parts into one.

Depending on the export setting you can keep all Parts in the grouping for further works or not. after this step just Import the Obj created into Poser.

note : It will also save the scaling when exporting ... so scale and position it already the way you wish having it ... else you will have to spawn the correct size through he Grouping tool after Import