Hi, you can do it quite easily. The front part of a marine torso fits on the front of the chaos warrior quite well.
Just needs a bit of green stuffing. Put a marine head on. The top of the legs needs a bit of a trim to fit in place too, be it isnt too much trouble. The arms look a little static however, but you can mix and match with cloaks and non cloaks so the unit looks more mobile! You also need to cut a shoulder pad in half and stick half on each shoulder and greenstuff some extra fur!
Thanks,
Tom.
P.S. I can pm you a message with photos of a conversion that i have done like this in a week or so, I am on holiday at the moment