User blog comment:Pualover/Overlord Power Level/@comment-14.202.187.93-20150914030000/@comment-213.113.51.4-20150917213807

Well even Ainz who is an undead said that once he reached lvl 100 he trained his strenght which in turn increased it. So even if they are undead it should be possible to increase stats. On the NPC's I am a bit unsure though seeing as NPC's are created to be in a certain way it might mean that they are static, in other words they are unable to grow in anything else then learning new things like tactics or such.