The more I think of it, the more I think that, if I were to need to build an undead army right this minute, for a Dark Ages, Medieval, or Renaissance setting, I had mostly only WGA kits to work with, and I had a wish to burn on it, I think I'd save the wish for a rainy day, and simply grab some boxes of WGA historical figures of the appropriate product line, and a box of WGA zombies and a box of WGA skeletons, for the heads, some arm swaps, and the occasional zombie body.
Rough up the shields and some weapons for weathering and decay/corrosion/battle-damage, apply rust, dust, mud, dirt, and decay effects liberally to the metal and cloth bits, and call it finished.
It's a your-mileage-may-vary kind of thing (as usual), but honestly, when we're talking about the amount of armor that soldiers from these periods of history are wearing, the only things that are going to set an undead horde apart from a living army are...
- Any human faces you'd see (vs. the undead versions); unnatural skin colouring for human faces (deathly pale skin, dark eyes, black lips - the usual gothic corpse-paint thing) will go a long way towards selling those as zombies, ghouls, or vampires. Zombie faces and skulls are even more explicitly undead. There's not a huge difference between those dark-age helmeted zombie heads, and skulls!
- Postures - slouching or lurching poses, clawing arms, lolling heads on broken necks. Some of these can be handled with zombie arms and awkwardly-placed helmets and heads; mix in some of those zombie bodies, one for so many armored bodies, and you'll get even further.
- Weathering, like grave muck, rust, nicks and dents in helmets, swords, shields. Most of this is just a routine part of painting up the figures, and most of the rest can be mechanically achieved with an Xacto knife and file, a can full of rocks to shake bits up in, and/or a cheap coffee grinder.
- The usual Warhammer schtick of slapping skulls and spikes on pretty much everything from shield devices to war banners and more. This can be omitted easily enough, but a little creativity and some of those spare skulls and bones from the skeletons kit can get you a lot of mileage here. (If I were feeling ambitious enough to splurge a little on bits, I believe the WGA Baron's War peasant levy and the WGA fantasy villagers probably have some Grim-Reaper-worthy scythes, pitchforks, flails and the like to mix in with the weapon bits to help "sell" the undead gimmick, too.)
And, we're not even limited to the dark ages/medieval/renaissance thing here, either: default WGA zombie and skeleton bits can go a long way toward "selling" as an undead army most of the WGA historical kits from the ancient Persians up through the Imperial Conquests Afghans and Boxers, through the world wars, into the Pulp Adventures moderns like the operators, using any of the above tricks to help with the illusion. Same thing with the Death Fields and other sci-fi figures, and the Classic fantasy human kits, and probably some of the demi-human kits (such as the halflings) as well.
Again, YMMV, and almost any WGA upgrade sprues that might appear would be a treat for me, but WGA has really provided us with a great resource in the zombies kit for building a variety of undead armies with some simple arm-and-head swaps, and the skeleton kit is useful for adding some skulls to the mix. Just about everything else rests on how you assemble and paint the bits, but that's a lot of the fun of this hobby.