I don't know if it's possible to add or remove traits to colonists after they are created but a mod which gives them the "hardened" buff after they see "x" amount of corpses would be nice. Maybe after seeing 10 dead bodies for example they are no longer affected by it.. this way your newer members are green and sensitive to it while your seasoned soldiers just accept it as a fact of war.
Alternatively a positive mood modifier for surviving a raid which more than offsets any dead body sightings might be an easier way to go. It should last half a day, at least long enough to stuff the bodies in the nooks and crannies around your base ;).
Ttm for a7 had a "survived raid" mood buff. That would be your best place to look as a starting point. Haplo also has a permanent "android" mood buff set for his MAI mod, I think you would need to go the route of a permanent mood buff instead of trying to add a new trait to already spawned colonists.
Meat of victory! ;D