What I've found it you can add multiple traits to the starting slave by just adding lines
startSlave.add_trait(slaveTrait)
startSlave.add_trait("Natural Beauty")
for instance works just fine. I don't know how to add traits to the player though since like you said they're not expected to have traits so I don't have some thing to emulate. I tried adding some to
func _on_slaveconfirm_pressed()
a couple of different ways but none of them panned out for me so If you've a method I'd be glad to steal it.
In a related note, traits added to the player work though generally only combat ones and maybe natural beauty have an effect, BUT they're also displayed in a couple places which makes them to me good not just for unnecessary optimizing but for flavor.
I'll fiddle with it more later as is my nature.