Vehicle modifictions are planned for sure, I like the idea of the extra makeshift bumpers/"zedstoppers" from oil drums along with the mesh covering a vechicle's parts that are vulnerable to a zed limb or two jamming up the stuff or them gaining entrance into the vehicle.
I'd say you'd need to weld most vehicle mods onto vehicles thou.
A lot of people love customization and that's another reason people will love a game, since it's not the same as every other game; something NEW.
I fully support vehicle customization like this, whether it's armoring it a little more to stop some extra bullets or to mow down zombies, or just not allow the zombies to hit the vehicle if you're sitting still.
Being able to have your own unique player character(not being the exact same as everyone else with customization) is one very nice thing, but having a customizable vehicle would be incredible. Personally I imagine a truck with plenty of bed room, and a motorcycle for quick point a to point b travel would be ideal, so having a unique truck and motorcycle would be awesome(keep in mind Im not sure if either of those vehicles will be or are added to the game