The restless Dead are from Border-town Burning does not include any restriction for hiring sell-swords So they can hire them like any warband.
The imperial Assassin can be hired:Any warband except Witch Hunters, Sisters of Sigmar, Orcs & Goblins or Skaven may hire the Assassin.
Any warband which includes Humans or Dwarfs may hire a Cathayan Merchant, including
Battle Monks of Cathay. For first look you could think that as the necromancer is a human it is a valid option. But as the majority of the warband are undead it looks a bit cheesy (imho). I say if the warband contains mainly human is better worded in this case.