I've been thinking about combining these two kits for years, and when I received a pack of the old out of production 'hunchbacked' warhammer fantasy chaos warriors I decided to give it a go.
From the WIPs you will see that the legs were bulked out with milliput. Regular Chaos marine legs looked spindly, despite the fact that the CSM torsos are actually a little larger than the chaos warrior's. 

 I also broadened the shoulders by inserting two circles of thin plasticard where the arms meet the body. I made these with a regular hole puncher. Also, the chaos warrior shoulder pads had a large lip that had to be carved off so that the backpack would fit. Even with this taken off, it was a tight squeeze as you can see below;


I then added borders to the legs and shoulders with thin strips of plasticard. This is my first time using milliput and plasticard, and it was a challenge to get it to do what I wanted.

 
I was trying for something along the lines of a marine in Pre-Heresy Mk 1 Thunder Armour; archaic. I gave him a nondescript paint scheme and, of course, removed all aquilas, skulls and chaos symbols as they would be inaccurate. Then again, I might stick some on any way.