How can I add an 'N/A' radio button to an existing row of 10 radio buttons?

If I already have 1 to 10 (unsatisfactory to great) but need an N/A button after 10, how do I do that?

It is not possible to add an "N/A" value to scale rating fields.  For your specific set up, I would recommend using a matrix field instead:

Set the row headers to your question text and the column headers to 1-10 with and NA option.

I set up a quick test form with a matrix instead of the scale rating fields.  Here is how it would look:

https://form.jotform.com/62625141763959

You can clone the form to your own account if you would like:

https://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL