I could add the trait later in console and save the game. It was still there after reloading (workaround)Regarding "fast_metabolism": this trait is race specific and only available to lizardman.
I'm still unable to change the portrait. I tried using party.party:getChampion(1):setPortrait("/Documents/Almost Human/Legend of Grimrock 2/Portraits/toorum.tga") but it didn't work.