Someone had a similar question on another forum, so I'm gonna add this here so that all the methods of adding a trait through editing can be found together.
You can edit the script files(.gd) to add a specific trait when you have a slave selected in game. You will need a decent text editor, most of the community uses either Notepad++ or Sublime Text 3 (the Godot editor can be used if the game is dropped into a blank project folder), and most likely the Debug mod. The editor will ensure that you can read the file and add newlines without crashing the game. The Debug mod will report any mistakes as errors rather than simply crashing the game. Though if you play through the errors then you may corrupt any saves made afterwards. If you mess up and are unsure how to recover simply replace the edited file with the original file from Strive's "backup" folder.
The best file to edit is ".../files/scripts/slave_tab.gd" in the program folder. "func _on_piercing_pressed" or "func _on_hairstyle_item_selected" are the best places to edit; they will trigger upon opening the tattoo menu or selecting a new hairstyle. Simply add a new line after one of those lines, then add a single tab(not spaces), and add this:
person.add_trait("trait_name")Replace trait_name with whatever you want; the trait names are the same as in game.