daremo wrote:
I've only seen video tutorials on switching torsos from one line to the next. To actually add the ab crunch articulation will require a decent amount of work but it can be done. You will have to cut the torso where you want it it to be articulated. Then you either need to fabricate the joint (ball joint might be the simplest to add here) or take the guts from a donor figure. Once you have the joint worked out, you'll need to go back and fix the sculpt so it looks nice. That's the basics of adding the joint. If I come across any tutorials, I'll link them here.
I don't think I've seen anyone do it who didn't have to cover up the cut line. You'd probably need two of the exact same figure to pull it off without an obvious seam.
More often than not, like MJ Tanner mentioned, I've seen people add a waist swivel or Marauder style ball joint at the waist where the belt line creates a natural transition.