"The Skaven are a race of bipedal ratmen that are so rarely seen that many deny their very existence. The majority of the man-sized vermin are slight of build and, if they abandoned their slinking haunched gait, stand bewteen four to five feet in height...Skaven are covered in close fur, save for their ears, muzzle, hands, and fleshy, worm-like tails. The eyes of the ratmen gleam red in torchlight and their mouths are lined with wicked teeth..."
First point to get out of the way, it's my belief the "Old World" of Warhammer and the gritty World of Greyhawk of Gygax's creation have always been contemporary publications and so hold alot of similarities in theme and mood. Heck, it's probably the reason why GW's Carl Sargent was brought over to add his grim touch to Greyhawk in 2nd edition.
Second, you can argue that D&D and thus Greyhawk does have ratmen already in the form of wererats. Wererats have been around since the beginning along with more lycanthropes than I can count. These however don't count in my book because they don't go far enough to be true Skaven. Wererats are cursed humanoids whereas the Skaven are a race-born unto themselves. Also, in D&D wererats are also not portrayed as the shadowy horde in waiting that is the Skaven...
"Although not regularly seen by surface dwellers, the Skaven are arguably the most numerous of all races. With a population of titanic proportions, the Skaven remain hidden away underneath unsuspecting nations."
The Skaven are more than a nest of monsters lurking in Greyhawk City's sewers ready to steal your purse. Yes indeed they can do that, but there is more to the Skaven than surface dwellers realize! The Skaven practice a blend of sorcery and science, a mockery of their hated enemy the dwarves and also quite similar to D&D's Derro. Thus, you'll see strange, warpstone powered warmachines and many lesser contraptions built to bring ruin and slaughter to the surface world. Other clans specialize in the spreading of pestilent plagues or the alchemical manipulation of their own kind into frenzied killers and altered into monstrous brutes like rat-ogres. Just as numerous and easily more cunning than D&D kobolds, the fastest of the Skaven of course, are masters of stealth and assassination with their poisoned weapons.
How's this work good for Greyhawk? Well as stated before they are similar to both Derro and Kobolds which are staples in Greyhawk but neither of which have the gravitas to be a true threat on an epic scale (unless you count the module Doomgrinder, yes I went there). The Skaven could be the power behind common wererat sightings, and a precursor to a much worse infestation to come. Muse over these scenarios:
Opportunistic Incursion: The Greyhawk Wars have just winded down and all the nations of the Flanaess are tired and low on resources. Suddenly bursting from beneath the ground of several key nations comes the coordinated attacks of a well-armed and endless horde of Skaven...
Congratulations On Clearing Zagig's Ruins: The heroes have killed all the monsters, taken all the treasure and returned to the city to celebrate. A year later the Skaven fill the void, scavenge what magic is left behind and stage a surprise attack on all nearby civilizations, perhaps even tunneling all the way to Greyhawk City itself!
A New Nation To Deal With: Iuz has been the omnipresent threat in the Flanaess for generations so who would suspect a new evil arising from below? Without warning things begin crawling out from under an unlikely region (such as Ket or Riftcanyon) and the Skaven soon have a foothold in order to attack both good and evil lands alike.
Vault of the...Skaven? Here's a twist for your campaign. The heroes have braved many miles of caverns and corridors and soon anticipate a big throw down with the dreaded, yet popularly known Drow. Only, they were too late. The Skaven got to Erehli Cinlu first and now they are going to track the PCs back to where they came from!