Can soldiers gain new traits?

Title says it all - and naturally I'm not curious of permanent injuries or level 5 passives from classes. :D

According to the Traits tooltip, that soldiers may acquire new ones during the story. I have however barely seen my soldiers get any, save for one occasion when a Legionnaire must have witnessed the death of his mate and he got "Indifferent", during an event.

Is there a way to naturally get 'useful' perks, such as Herbalist or Housekeeper?