Probably the only way to fix shoulders in Poser would be with joint-controlled-reverse-morphing (JCRM). What is JCRM, you ask?

Basically, you take the affected body parts in their bent position into a modeling program, correct their shape in this position, then apply this as a morph target. Then you apply the bent plain versions of each as a morph target, set this "plain" morph to -1 while keeping the new morph set at 1, and then spawn a new morph target from that. Then use the bend to control the morph dial. Simple, no?

I call it ..reverse morphing because you are basically reverse-engineering the bend pose with a morph target. Shoulders are one thing that isn't easy in any program.