Sim, você pode polimorfar uma criatura em um NPC de lançamento de feitiços
O texto das regras é bastante claro sobre isso.
You transform the creature into a different creature ... The
transformation lasts for the duration, or until the target drops to 0
hit points or dies. If you concentrate on this spell for the full
duration, the transformation lasts until it is dispelled.
Creature into Creature. If you turn a creature into another kind of
creature, the new form can be any kind you choose whose challenge
rating is equal to or less than the target’s (or its level, if the
target doesn’t have a challenge rating). The target’s game statistics,
including mental ability scores, are replaced by the statistics of the
new form. It retains its alignment and personality.
The creature is limited in the actions it can perform by the nature of its new form, and it can’t speak, cast spells, or take any other action that requires hands or speech, unless its new form is capable of such actions.
Se você quiser Polimorfizar True uma criatura de nível 12 ou CR 12 em um NPC arquimago, CR 12, você pode fazê-lo, e ele poderá lançar seus feitiços.
Archmage (SRP p. 395)
Medium humanoid (any race), any alignment
STR 10 (+0) DEX 14 (+2) CON 12 (+1) INT 20 (+5) WIS 15 (+2) CHA 16 (+3)
Saving Throws Int +9, Wis +6
Skills Arcana +13, History +13
Damage Resistance damage from spells; nonmagical bludgeoning, piercing, and slashing (from stoneskin)
Senses passive Perception 12 Languages any six languages
Challenge 12 (8,400 XP)
Magic Resistance. The archmage has advantage on saving hrows against spells and other magical effects.
Spellcasting. The archmage is an 18th ‐level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). The archmage can cast disguise self and invisibility at will, and has the following wizard spells prepared: {bolding mine, spell list edited out for brevity}
*The archmage casts these spells on itself before combat. (mind blank, mage armor, and stoneskin are listed as aserisked spells)
Veja esta resposta para uma descrição detalhada do que está no bloco de estatísticas de uma criatura.