I am in the process of creating a dragon for a campaign I am writing. I am planning on adding 5+ levels of sorcerer on to an existing blue dragon. My question is, since dragons cast as a sorcerer, would these new levels of sorcerer stack with the dragons existing levels of spellcasting or not?
I would say yes, pretty much all other creatures stack their casting with like classes. The other question is whether it would count as a key class for the purposes of CR, but within the scope of your own campaign, that may not matter!
I would say that it counts as a key class because it enhances the dragon's existing abilities as a caster, which is the driving concept behind the dragon.
Yes, it does stack; dragons explicitly have a sorcerer casting level.
Yes, it does count as a key class; the Bestiary says so.
As I recall, adding levels of Arcane caster to Dragons doesn't really do much for them. This seems both counter intuitive and sad.
I don't remember where I read this but other classes are better for Dragons: barbarian, Paladin and Thief.
This was some time ago; maybe levels of a divine caster would be better.
Bestiary appendix, page 297, Adding Class Levels, Step 2: Add Class Levels, paragraph 2:
"Next, add the class levels to the monster, making all of the necessary additions to its HD, hit points, BAB, CMB, CMD, feats, skills, spells, and class features. If the creature possesses class features (such as spellcasting or sneak attack) for the class that is being added, these abilities stack. This functions just like adding class levels to a character without racial Hit Dice."
As I recall, adding levels of Arcane caster to Dragons doesn't really do much for them. This seems both counter intuitive and sad.
I respectfully disagree. Caster level is one of the most versatile features that you can add to a monster, especially if that monster already have spellcasting levels.