WalkerSim makes such a huge difference to the game that you have to wonder why this isn't part of the base game.
WalkerSim can be configured to be exactly what you want it to be, occasional large wandering groups? check, individual zombies everywhere? check, a combination of both? check.
Get it, single or multiplayer and you won't go back to vanilla!
<csv xpath="//prefab_spawn_adjust[@partial_name='trader_jen']/@biomeTags" delim="," op="add">forest,snow</csv>
<csv xpath="//prefab_spawn_adjust[@partial_name='trader_jen']/@biomeTags" delim="," op="remove">burntforest</csv>
...then generate a new world. (Make sure all biomes have traders in them.)
Don't blame me if she freezes to death in that outfit. :)