I've used modified D&D rules in the past for Sword and Sorcery games, but they don't have the same feel Rune Quest has. The game is gritter and more geared towards the low fantasy style of play needed for Sword and Sorcery. I highly recommend Lankhmar it's my favorite setting following Robert E. Howard's setting.
I went and dug up the Black Magic rules for you, here is a brief synopsis.
It functions almost identically to Sorcery found in the Mongoose Runequest rule set. Each spell counts as an advanced Skill. Then there are the four Manipulate skills: Magnitude, Range, Duration and Target. The additional rules are Sympathy, Agony, and Hatred. The better a Sorcerer knows his victim, or has a personal effect or body part, the easier it is for them to cast a spell on them. A Sorcery is also able to burn off hit points giving them a +5% to their Spellcasting and Manipulation rolls. Finally the more a Sorcerer hates his target the easier it is to inflict harm upon them with Black Magic.
Corruption works like detrimental mutations from Runequest. The energies warp the Sorcerer's body, mind and soul. A test to see if a Sorcerer gains corruption is made whenever they learn a new black magic spell, casts a black magic spell, kills someone with a black magic spell (two corruption tests are made in that case). It's a 50/ 50 chance that the Sorcerer will gain a corruption feature, with the following modifiers. Every 5% used of Agony in spell casting is added to the chance they will gain a corruption feature. If they use Hatred they suffer a 5% penalty per level of spell Magnitude to the test.
Looking through the Conan book I'd recommend Agony be used to represent Ritual Sacrifice/ Torture to gain arcane power.
Definitely going to check out this book.
My favorite set of rules for magic are the one in Cinematic Unisystem. They're very different from the others I've seen. You make spellcasting rolls instead of having points or spell slots. Casting too many spells causes fatigue. If I were to adapt it to D&D, I'd probably use the existing fatigue and exhaustion rules.
Not sure how I would put it in RQ if I were to try. But RQ Lankhmar may be a better fit for running Conan.
Out of curiosity, Why do you prefer the simplified cultures to setting-tailored ones?
Funny you mention that, but Cinematic Unisystem is my favorite set of rules for magic too. Right now I'm working on a modified version of Barbarians of Lemuria using a variant of the Cinematic Unisystem. Another game that comes close, and is another of my Sword and Sorcery favorites is Desolation. It has a free form magic system similar to the Unisystem.
In regards to your question... I just feel the simplified cultures cover the range well. Cimmerians are obviously Barbarians, Picts Primitives, and Stygians Civilized (debatable).