I want to add a custom dropdown list on my `account.invoice.line` which can have entries added (I don't need edits, but having it doesn't matter). I tried with a selection and reference field, but it doesn't let me add any new entries.

Do I need to create a custom model and a many2one relationship to it for the create and edit ability?

Also, I think I figured out where to change for printing the custom fields, but I am asking here to make sure.