short answer: yes you can.

longer answer:

the game allows you to customize both your faction and your avatar/leader of your faction. one of the premade factions is close to what you describe: the "Magnar" faction is a race of slavers that get extra population when the defeat human enemies and when they raze conquered cities; and they can build slave armies in addition to their normal units. their leader/sovereign isn't really designed as a necromancer, though. his abilities are better suited for a stright damage caster (think "Fireball spamming Evoker"/"Destuction mage"). You could probably replace him with a different leader that is more focused on summoning. The trait tree for the mages of Empire factions contains a line of summoning spells for summoning undead minions, and Empires can utilize Death magic which is similar to the school of Necromancy in D&D for example, with spells that weaken or instantly kill their victims.