I want to make zombies unable to go through doors, so they'll stop hanging out boringly at home and survivors can take shelter in buildings without me having to manually secure the perimeter for them. I've been able to accomplish this in a really crude way by making a zombie caste and then setting every exterior door on every building in my zombie apocalypse test town of Moonlight Falls to be locked to zombies; the result was as desired, but manually locking zombies out of every building in the whole town was a drag and I'd rather not do it again, so I want to try and make all doors automatically be locked to zombies (or whatever other filter) instead.

If that can't be done with existing mods, I might try to make something myself; I finally have a computer I can mod Sims games on and am always down for learning how to code new things. So I just want to know, 1) is automatically applying locks to all doors in a world possible already, and 2) if not, is adding this feature myself likely to be so challenging it's not worth trying? I know the second question might be tough to answer conclusively, I'm just trying to get a general sense before I go for it.

I don't know of a mod that gives you global options, with Gohere you can disable all settings you made to doors global I think this is for a special time, temporary.
I think yeah - you need to set Door locking to all doors in your town single by single.
(this might can slower the game performance if there are too many the mod must check everytime a sim will enter)

Or try this EA item Barrier to Entry System https://store.thesims3.com/productDetai ... SIM3:62502

You might can try to use Transmogrifyer to find an object which fits to the places and copy the script from the Barrier - this is experimental and must be tested http://modthesims.info/d/592332/the-tra ... anger.html

EDIT:
Or if you fence all lots you only need to lock the fence gates, this will give the sims better chance to escape the zombies