Is your issue with the visual display of the option, or keeping your analysis clean? If it's not a requirement to set the "Prefer not to answer" option apart from the Likert scale choices, you could add it as the last option in the list, and use the Exclude From Analysis option so it is kept out of the results and reports.

Thank you for your answer. I think it is more of a graphic thing. Aka how could I make the last choice a separate button (or something like that underneath)

OK, I see. There's not really a built in way to do that in Qualtrics. It's not the most elegant solution, but you could trying setting this up as a side-by-side question, and use custom validation to make sure they only answer the first or second column, but not both.