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I recommend for additional inspiration for this idea that you read World War Z: An Oral History of the Zombie War by Max Brooks. Its probably the best fiction book I've read in a while and I guarantee that you will find a wealth of resources there.

Edit: A bit of info on the book:
Zombies are created by a virus called Solanum.
Solanum is contracted by the contact of bodily fluids with the virus, usually through biting or the splatter of infected matter into an open hole in the body or wound. Ingestion of infected flesh causes rapid death, but isn't followed by resuscitation.
Once a human is infected, they will show fever like symptoms as the virus works its way through the body to the brain. Once it reaches the brain, which can take up to a day depending on where the infection first occurred. (ie one man in the book receives an infected heart in a transplant and dies within the hour as the blood delivered the virus to the brain almost immediately, but others who were infected in peripheral body parts would go home, die and be resuscitated). An hour after death, the virus will establish its control over the brain and the corpse will resuscitate.
Characteristics of a zombie:
These aren't any kind of magical super zombie. Its physical strength is the same as the corpse it infected. Due to the damage caused to the brain, zombies operate purely on the instict to devour living flesh. They lack the coordination to perform tasks such as climbing a ladder, or opening a door (though they can certainly break them). They are completely unthinking, so they cannot learn or coordinate. They just stagger (slowly due to the damaged brain), thoughtlessly, fearlessly, and tirelessly (they are unaffected by fatigue) towards their prey once detected. Zombies have somewhat heightened senses, as they rely on all of them equally. Zombies can only be destroyed by destroying the brain (eg headshot, blow to head, stabbing the brain). Damaging the body will not kill a zombie, but it can slow them down permanantly, as they cannot regenerate any damage to the body, no matter how minor. Zombies will also freeze in sub-zero temperatures, which has the double effect of immobilizing them and slowing down their gradual decompisition which usually takes a decade as the vast majority of bacteria and other decomposers are killed upon contact with the zombie virus. When thawed, a zombie will simply continue doing its thing.
In-book human response:
Originally, the zombie outbreaks were either covered up, ignored, or acknowledged, but not really dealt with. When it was acknowledged. This meant that the outbreak was able to slowly grow until they grew to the point that it became a global problem with almost every nation on the globe on the verge of collapse from the zombie infection. Eventually, the worlds countries adopted a plan that would involve a select body of "valued" citizens for the war and rebuilding effort as well as the government and large portions of the nations remaining military falling back to an easily defensible springboard from where they would regroup, rebuild, and eventually reconquer. The remaining citizens and military troops would fortify themselves in "islands in the zombie sea" in order to divert zombies from the safe zone. Many would be overrun.
Aside from zombies, some humans became threats to the rest of the race. For example, certain humans were driven insane by the situation and either just died or began to act exactly like zombies. These could not infect humans and were easier to kill. Also zombies can tell the difference and will kill them.
Effect on gameplay in the mod:
Every last zombie will have to be killed in order to win a battle, and the same on the campaign level.
Soldiers felled in hand to hand combat with the infected will become zombies.
Towns attacked by zombies that are undefended will have their population converted to zombies.
Settlements and forts besieged for a long period will have insanity cases begin to occur and troops and citizens will either slowly die or imitate the zombies.
Siege battles will be somewhat easier because zombies cannot scale fortifications, only slowly destroy them. However, you will have to kill every last zombie to win, though in order to allow for multiple turn sieges, for every turn until you either destroy the zombies or they destroy you only a portion of the army will attack (like waves)
The outbreak can begin in several regions, infecting citizens and troops alike, and will spread from there based on troop and agent movement, and trade (so islands are by no means safe havens, as the infection will find a way there eventually, unless you self quarantine, which will be difficult as you would not really know whats going on until the first serious outbreak occurs (maybe first province overrun?).
Zombie armies are slow, but immense and will break most units with first contact in the first few engagements. (due to the OMG WTH BBQ IS THAT??!!! factor)
In winters above the snow line in the game or in mountains, zombies will freeze, allowing for free movement and the easy destruction of zombie armies. To compensate for this obvious advantage, all months but winter could have two turns and winter only one.
The campaign is a collaborative effort. You win when every zombie in the earth is (re)dead, not just in your own lands

Just some ideas that I think could help any one aspiring to create this mod get started and get some structure that I took from the book.