Well! 
- Gulâdan quite literally created Death Knights ⌠He took the slain warlock souls, threw them into dead human corpses and reanimated the bodies to be functional.
These warlock âDeath Knightsâ were also some of the first necromancers (the ones youâve been mentioning lmao) that were seen in Warcraft! 
Source: Warcraft II
- The Shadowmoon clan necromancers are also referred to as warlocks, using the void to bind souls and control the undead.
Source: Warlords of Draenor, quest - âSecrets of Soulbindingâ
- Warlock rituals can cause an upheaval of souls, reanimated corpses and consequentially manifest or summon spirits from the realms of the dead.
Source: Legion, when Gulâdan severed the soul of Illidan from his body.
Additionally:
Fel magic reanimating the dead is hardly anything ânewâ either, as seen in the Sunwell raid when the residual felblood from the slain Pitlord reanimated Madrigosa into an undead fel-drake.
So there ya have it! 
Thanks for asking the question âWhat lore?â â So we could reinforce what was said earlier.
If Blizzard present warlocks with necromancer skins, it will be a thanks to you indeed 
I made a suggestion for a necromancer warlock-skin thread ages back too â