I am also interested in this. I talked to Satain about this and this was the response he gave me when i mentioned his Hogue AK-grip:
All you need is an Hogue AK style pistol grip an exactoe blade/dremil and a yellow marker. Hilight the area that needs to be removed and dremil/cut it out.
Hope this helps a bit. But if you are interested in different grips, you can put an AR styled grip on your 858 as well. I am going to put a Magpul MIAD grip on mine, but you will need a Brownell's AR to AK grip mod (~$7).