All the spawn points for these missions are in fixed locations. For the Starving Citizen and Wolf Attack missions, once you complete them you can walk away (make sure it's well off your mini-map) and then walk back and they will respawn in the same exact spot. For the Convoy missions, I never had much luck with getting them to respawn. It took me a while of randomly walking around on the roads and hoping as I approached a spawn point that it would pop up. I had better luck getting them to spawn by not walking directly on the road, but walking off to the side of the road. Not sure if that actually contributes to anything though.
A quick and easy way I've heard about that works for convoys and starving citizens (wish I had known about before) is to wait until you get the "Mission Complete" notification after completing them, then just Pause and Quit the game. Load the game back up and you will spawn back in the same location next to the mission at hand and you can complete it again. Waiting for the game to load will probably take less time than running around waiting for them to spawn.
As for saving citizens, I've heard that some people have had luck with only attacking one or two of the wolves, then letting the third wolf maul and kill you. You will respawn there with all three attacking again, and it will consider the previous mission as complete, allowing you to do it again. I can't confirm this, though, as in my experience it only gave me the "Mission Complete" notification after I had killed all three. Worth a try though.
Convoys generally spawn more often in or around the towns in the middle of the map.
Wolf Attacks tend to spawn near the roads (not on them, but close by) away from the towns, so don't bother exploring off into no man's land.
Hungry Citizens spawn, literally, everywhere. If you're having trouble finding them, then you're probably not playing the DLC.