I've noticed that there isn't a whole lot in the way of spells to use for our enemies with the new expansions and I'm wondering if anyone has made or converted older spells to new edition for the Necromancers in the game? I really wish they would have gave some good spells for the different NPC races........

Which expansions do you have?

For me the dark magic/necro, and several Tze/Nurgle spells are plenty for me to make potent adversaries.

Also I'll often give their enemies access to colour magic, because once you cross the barrier to dark magic, you can use anything you want really, of course at the cost of more corruption

You have some nice Dark Magic, Chaos, Tzeentch & Nurgle spells Ranks 1 to 3 in WoM + SoF, some Chaos, Shaman / Dark Magic, Necromancy & WAAAGH, Shaman Ranks 1 & 2 in TGS, some Chaos, Skaven Ranks 1 & 2 in EoN. They are plenty to customize bad guys

Those do not appear in the Creatures (Guide &) Vault.

The upcoming Witches Song should have some Hedge and Dark spells, I believe.

In a pinch, you could also take a few existing spells from other orders and faiths, perhaps some of the more obscure ones, and reskin/retheme them to give them a necromantic or dark effect instead of fire/weather/animal etc.