Part of the answer to the puzzle first. Here are the specific pieces I used.
The axe has had the blade removed and the forearm has been separated from the hand. The pointed metal plates are the 'teef' from the boss pole. How will they be used? The actual Ork toof for the base of the axe haft was also taken from the pole. Don't get too attached to that - why not? The spiked pauldron will help make the new axe head.
Here's how they went together.
Yep, the metal plates are a warm-up for the fuller bionik fusion of Ork and machine. If Orks are so tough and accept bioniks so well, why wouldn't they go beyond warpaint and tattoos for decoration and simply bolt armour straight into bone? Think of the pride of this Ork in the mob. A few teef and the dok has him feeling like a space marine.
The largest plate had the panel at the base reshaped into a hexagonal bolt head, which went surprisingly well. I added extra chipping to the edges of each, to suggest plenty of blocking of incoming strikes, as if they were used as an extended shield might be. The plates were then glued directly onto the bulges of muscle, one per bulge.
I'll go a step further towards full bioniks with the left arm, the one with the shoota, and hopefully much further with the body.
The surfaces where the forearm and hand meet were shaped to allow the axe to be held at a shallow angle. I want the Ork to be looking away from the direction of movement and along this arm, pointing as if directing or threatening.
For the axe head, to the inner surface of the spiked pauldron I added two flat squares of greenstuff where the bolts sit on the outer surface, to suggest brackets, as if the bolts were fixing the pauldron to the haft. I built the blade back up with greenstuff to the point at which the inner surface would just meet it and sit in the right position. I could jiggle this about plenty while the greenstuff was drying so it was easier than it might seem. The surface wasn't especially smooth at this point so when dry was shaved flat.
One of the greenstuff brackets was covered at this point more than I would have liked, but as the square was still wet, the two pieces of greenstuff merged and I conceded the point. As can be seen, I then extended the ends of the pauldron around the haft and gave it the same rough edging. This was tricky work for the usual reasons - the greenstuff has to be wet for easy shaping, but when wet it sticks less easily to the fixing point. Patience and concentration is the key. Slow, steady progress and careful application of force in the right direction every time. Calm.
A small ball of greenstuff was then pressed onto the point at which the haft would emerge, then smoothed and the edge squared off to give a very low cylinder.
As for the toof, it vanished. Unsurprisingly I'd been in danger of losing it right up to that moment. Where it is now, who knows? The replacement was made in just the same way as the one for the stikkbomb arm and head, and connected in the same way as to the stikkbomb.
Any questions, I'm ready. Remember, it's the zen of modelling - speed is for the fast.