Sim, se o monstro puder conjurar mágicas "como um [x] th nível [y]", então, os níveis nessa classe aumentarão o nível de conjurador do monstro. Veja o SRD em melhoria dos monstros:
A spellcasting class is an associated class for a creature that already has the ability to cast spells as a character of the class in question, since the monster’s levels in the spellcasting class stack with its innate spellcasting ability.