The process is semi-time consuming but it is worth it.

1. You should select the captians you want to turn (6 at the most) and put them as assualt leaders.
   
2. Go into an online ranked seige and once you have captured all the points and before you go into the overlords room start smaecking the ♥♥♥♥ out of the ones you want to turn until they bleed out
   
3. They wont betray you while you are doing it becuase its turned off in online seiges. Once they bleed out ressurect them in the army screen while still in the seige.
   
4. Once you have finished off the overlord and go back to your regular world the ones you turned will still be there but with their personality and tribe while still keeping the undead stuff
   
5. They wont be able to immediatly level up but using a reasignment order on them will allow that.

And thats it, have fun creating your army of undead who arent husks of their former selfs. If you have any issues post them in comments so I can try and resolve them.







BTW a quick thank you to †Genesis†
because of his guide I found out about this glitch
http://gtm.you1.cn/sharedfiles/filedetails/?id=1171546721

Also here is a link to the video Uruk's Hallow made after I told him about the glitch!
https://www.youtube.com/watch?v=A2TWv5jHVQs

UPDATE: Sadly with the 1.18 patch this glitch is no longer working. But your undead captians from before the patch should still have their personalities and level up.