As part of my Kingmaker campaign, I've stolen/borrowed from a LOT of sources to add NPCs and flavour to the hexploration. Mostly from my collection of Dungeon & Dragon magazines, as well as a few published adventures. In case it inspires anyone else, here's what I used & how I adapted it.
Throne of Iuz - Dungeon Magazine #118
There's a web enhancement for this issue of Dungeon with all the artwork from this adventure - I really recommend taking a look. The whole thing looks grotty and nasty, so perfect orcs!
In a pretty big departure from the Kingmaker books, I put a small kingdom of orcs into the southern part of the Narlmarches. There were a few reasons for this - I suspected that exploring hex by hex was going to get a bit tiring by this point, so it replaces a lot of the random encounters. It's been a while since I ran a game which had nasty, brutish orcs in it. I replaced the lizardfolk village where Tig Tannerson is held captive in the Greenbelt with an orc outpost. This was largely so the PCs could go all righteous fury on them without having to worry about there being kids, the elderly, etc. there.
Throne of Iuz is about an Awakened Giant Toad (the handiwork of Iuz the Old) named King Bog, who takes control of an orc tribe under the pretence of being divinely chosen by Gruumsh. The Hant tribe of orcs live around an ancient elven tomb called the Serpent Mound, and raid the surrounding lands using chimera mounts. King Bog has two disguised slaadi enchanters working on an iron throne for Iuz which will magnify his power, and the raids are escalating to generate the resources this requires.
In Kingmaker, Iuz becomes Nyrissa. King Bog is a fat toad swollen and woken by her magic as part of her plan to bottle the Stolen Lands. The throne is now wood, but will help bolster her 'claim' to the land, and the Hant are used to harry the PCs kingdom.
Instead of One-Eyed Gruumsh, the Hant tribe was now a cyclops cult revering a Great Cyclops, because I read the entry on them in Varnhold Vanishing and they're awesome. King Bog ate their patron, but then had their chief druid ritually put out one of his eyes and claimed that he now had the foresight of his forebearer.
Putting in the Elven tomb also fits with the ruined tower of the Dancing Lady in Book #2; once Elves tried to settle the Stolen Lands, and it didn't fare much better for them than anyone else.
The Slaadi became Redcap servants of Nyrissa, glamoured to look like Orcs. Redcaps aren't exactly known for being great crafters, but they're suitably brutal to fit in amongst orcs, and my PCs hate them since encountering them earlier in the campaign.
How it played out:
I was dropping references to the Hant from Book 2, when the PCs rescued Tig, but they didn't become that relevant until the early parts of Book 4. A few mentions of Orcs on the PCs borders in the Narlmarches, a few trappers or loggers complaining, enough to generate a little Unrest.
The PCs decided to see if they could shut this down personally before it became a serious problem, and wound up meeting a Hant hunting party who were after the Hodag from Book 1. Turns out that thing just keeps reincarnating - I went with the 'born from a sacrificed bull' origin story, with First World influence meaning that it's remains will self-immolate and discharge a new Hodag within a year if killed. The players made some awesome Diplomacy rolls and got the hunters to bring them back to the Serpent Mound, where they met the orc General, but not King Bog. A bit of tense negotiation made it clear that the orcish "might makes right" philosophy was going to bring them into conflict sooner or later, and they left knowing the sooner or later conflict was coming.
They decided to nip this in the bud. Courtesy of the party Wizard (Invisibility, Flight and Teleport) the whole party got inside the tomb without alerting the orcs, and killed King Bog. This was a very dynamic fight - Bog jumps around a lot and crushes/swallows people whole, and with a few levels in Horizon Walker he's quite tricky to lock down. He's backed up by a Wildshaping Orc druid who favours killer plant forms, the druid's Giant Boar animal companion, and two Elven Caryatid Columns that attack any humanoid not wearing the emblem of the Serpent Mound.
Since the boss room has some huge fungi growing around the edges, I added a few fungus leshi who were doing grunt work for the Redcaps. They joined in the fight but were far too low level to do any damage, and once their bosses were killed they just stopped fighting. They were able to provide a bit of exposition, and were fun to roleplay out with a "look mate, I just work here" ethos and total disinterest in either side of the conflict. The players also enjoyed the image of a two foot tall mushroom monster waddling up to their tank and futilely trying to smack her with tiny shovels.
Once the PCs had killed the toad, they just walked out of the tomb, told the orcs what they'd done and stared them down. This was brazen enough to work and to ensure a year or two in which the orcs wouldn't want to mess with them, which ultimatly led to the Hant being available as mercenaries during the war with Pitax.