This is a quick tutorial on how to add elbows to SS Rampage. If you have any questions just ask  
What you’ll need: Dremel (with cut off wheel, sanding bit and grinder bit), ruler (for keeping everything even), and superglue.

First you must cut the LEGO pieces. Cut at the red lines and keep the non red pieces. Also cut of the studs.


Now cut the whole arm at the red lines. Now cut a slot for the dark grey piece and insert the longer side into the slot. Be sure the priece is upside down.
Cut out enough space for the light grey piece to sit in like this.
This is how they should look when done. You may need to sand the LEGO pieces to make sure they sit flush with his arm. Now you are done!

 

These are the pieces that you will need and this is where you can get them. BrickLink - Search result for 14418 - BrickLink Search

 

I really like to use those pieces, too  
Could you show pictures of the result (added articulation/poses), please?

 

He now has a little over 90 degrees of bend. And basically has a wrist swivel. Here are some poses. (͡ ͡° ͜ つ ͡͡°)