Disclaimer: It's been something like a year since I last looked at this. I don't use this mod, and frankly you shouldn't either. It uses very primitive methods, and adds/remove spells from all kinds of actors like a maniac. Not to mention that from a lore perspective, only the PC and a few exceptional individuals should be able to manifest and consciously wield the powers conferred by any particular birthsign (with the possible exception of the Atronach sign cursing all those born under it with stunted magicka).
You may crack it up in the CS and look at its scripts to maybe learn something new, or not, but its best not to actually use it in a game you care.

Enemy actors use powers seems to be a similar mod, I would never have guessed my idea was not all that new.

Introduction
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So I was wondering, "friggin' arrays, how do they work?", or something like that, and realized that the best way to learn it was using them.
Because the OBSE command GetBirthsignSpells was sitting there, lonely, with no mod making good use of it, it appealed to me as the perfect candidate. "A mod to add birthsigns to NPCs" sounds easier than it really is though...
Many, many, MANY curses, arrays and hours later this piece of imagination came to fruition. Featuring random birthsign assignment to any NPC, humongous scripts with a NOTICEABLE performance hit, fairly safe system to remove or change the birthsigns on actors, and some more (ok, more like less, because when you are adding spells directly to actors, the less the better)

Details
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Upon loading a game, this mod creates three arrays: one contains the spells from the vanilla 13 birthsigns (maybe more ;) BUT with the spells defined by any 'birthsign overhaul' type of mod you might have like bg's Race Balancing project or tejon's Birthsigns Expanded, another contains actors that should not get a birthsign automatically, and another is for Shadowscales (I might refine this later).
The mod works by spreading tokens among any actors catched by GetHighActors obse function, basically anyone not entirely dead in your same cell. You can set exceptions, like preventing essential NPCs, guards, persistent or non persistent actors from getting token, check the ini!

Upon being add, the token generates a random number from 0 to 12. Each number from 0 upwards is a birthsign, in alphabetical order (0 for Apprentice, 1 for Atronach...). It then checks that the actor has no spell from any birthsign already. If the check is successful, the spells in the birthsign get then added to the actor. Poof! It is EXACTLY like magic!

When the NPC dies, or the clean option in the ini is set to 1, the token removes the birthsign and itself. You can also do this manually (keep reading)
By default, this mod excludes some NPCs from vanilla Oblivion that already have birthsign spells, or don't really fancy one. Martin, Uriel, Mankar, Mannimarco, Burd and Jayred Ice-veins from SI are the ones provided by default. It also excludes daedric NPCs (because 'birth' doesn't apply to daedra)
What are the Shadowscales? Argonians born under the Shadow birthsign. By default any argonian in the dark brotherhood is treated as a shadowscale by this mod and gets the Shadow birthsign. All the argonian Shadowscales from Ruined Tail's Tale, including Ruin himself, will get treated as such too.

The exception and birthsigns list can be edited in the .ini, where you'll find templates for both. High knowledge of things like formIDs and being able to look up for them is required. You can add additional birthsigns to the random selection list, and NPCs to be skipped by the mod.

So you didn't (or did) include your favorite companion in the exception list, and want him/her to have a specific birthsign. When debug mode is set to 1 (either in the ini or in game by typing 'set aaBirthsignNPCQ.debug to 1'), not only will your console be flooded with useful? info about what the mod is actually doing (best to use ConScribe to check it in depth), you'll also get the 'Read Hands' lesser power.
Cast it at an NPC. You will have the option of assigning one of the default birthsigns (Sadly I can't include custom ones in the selection with security). They are grouped under aspects (The warrior includes warrior, lady, lord and steed; the thief includes thief, lover, shadow and tower; the mage includes mage, apprentice, atronach and ritual; the serpent is a bit loose). Or you can remove any birthsigns on the actor and ensure he remains birthsign-less with the 'Nothing' option. Cast a second time for allowing the actor to get a random birthsign.
Read hands works only on friendly NPCs, and yes, you can cheat it by giving everyone you come across a crippling or overpowered birthsign should your game have edited one as such, I trust you'll use it wisely :)

Install
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Drop and activate. Ensure that the 'Birthsigns for NPCs.ini' is in 'Data/Ini' folder, and tweak it to your liking. (Possibly) enjoy.

Uninstall
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THIS MOD IS HARD TO UNINSTALL SAFELY, though not impossible. Ensure that the aaBirthsignNPCQ.clean is set to 1 in the ini and play for a good while, 30 min or so, and birthsigns should get removed safely enough. Even better if you slaughter some bandits and marauders, since once the birthsign is removed from one NPC, it is removed from all his references in the game.
YOU SHOULD ALSO DO THIS if you want to start OR stop using a birthsign overhaul type of mod, like bg's Race Balancing project or tejon's Birthsigns Expanded, to ensure no leftover modified AVs by spells and abilities remain on actors.

Changelog
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0.5 - Initial release

0.51 - Many bugs fixed (go figure), custom addons to birthsign and exception lists actually work now

1.0 - No longer depends on Oblivion.esm (YESSS), fixed and streamlined the customization options for mod added actors and birthsigns

Credits
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Thanks to Bethesda for providing me endless challenges.
Thanks to the OBSE team for including hardly-used functions for the scripting romantics like me (ugh, that sounds... unmanly...)
Thanks to you for downloading this mod and being brave enough to use it!