There are few ways to do it
- Add a small side property to custom spell. For instance, if that a buff spell, you might want to include the 'healing effect' of 1pt to that spell. NPC will tend to cast them when they are wounded. Add a small hp dmg to the spell, NPC may try to use in like any other destruction spell.
- The better way yet with no side effects is via Scripting

. But very complicated......You can script 'force' NPC to equip that spell of yours, and he may use it

EquipSpell(Spell akSpell, int aiSource)
Or even better if you could program the AI to use that spell in some combat situation