Darth Vader Stance

Well, what you are up to implies changing the posture of arms, legs and hands.
This is not too difficult but if it's your first attemp at doing a conversion I'd try on a scrap figure first.

Basic technique is the following (let's consider an arm, for example):

• Cut the arm at the ellbow (knee if you are working on a leg, pulse on a hand etc.), removing quite a bit of the ellbow itself.
• Drill a hole in each art of the arm to insert a metal wire (brass or soft steel); this way you can bend the arm to the desired position
• Fill the gap with putty and recreate folds an texture of the cloth.