While I'd like to agree that they were the imperium of man lets look at the facts;

 While they share a certain architecture both have distinct philosophical differences

 - the imperium of man does not like Xenos now while that doesn't mean much yet what we know of the Necromongers is that they are on eof the 4 ancient races (not necessarily man), yet they seem to take any humanoid who is both willing and worthy.
 - the imperium of man may be agressive in it expansion and it treatment of outsiders but it home rule is somewhat different; while you may be subjet to routine house searches have you data searched and maybe even burnt as a heritic (if your lucky) you know this isn't random infact the amount of paperwork needed to go through the ministerium before they do almost anything is ridiculous. so not a rule by might but bureaucracy.
 - while both are heavily religious one worships a man who has become a god (the god emperor of man) this man lives still (well certain people have seen that his life support has been shut down but the imperium as a whole doesn't know this) and is used as a rallying point to stay to what is human and breed an intense hatred to any who speak against them. whereas the Necromonger worship chaos and death.
      
Necrons maybe.... a primitive version


 So in closing not the imperium of man maybe a cult of  Korn  with some slaanesh worship  thrown in for fun.
 

  And yeah on that note I'll say maybe but I've been looking closer at Necromunder the table top that takes place in a hive city with rival gangs whiles still in the 40k universe it seems to fit them better and may also explain the inspiration for the name.