It is not possible to add an N/A option directly in the NPS question because the scale is predefined.

There are two workarounds:
1. Use a matrix question with a scale of 1–10 and add an extra option for N/A.
Reference screenshot: https://app.screencast.com/qORRI3rxMx3GY
Help Article: https://www.questionpro.com/help/matrix-multipoint-scale-question.html

2. If the NPS question must remain in the survey, add a multiple-choice question above it to confirm from the respondent if the scale is applicable. Based on the response, apply Dynamic Show/Hide Logic to display the NPS question only when the respondent selects Yes. You can also keep Yes selected by default so that the NPS question shows initially.

Steps to do so:
i. Add a Single Select question and add the options "Yes" and "N/A".

1. ii. Add a NPS Question below it.
2. iii. Keep the option "Yes" selected by default to keep displaying the NPS question until N/A is selected.
3. iv. Apply the Dynamic Show/Hide Logic on NPS question to display only when "Yes" is selected in single select question.
   
   

Reference: https://app.screencast.com/eHLCmurvbqNw0
This will make NPS question visible along with the respondent having the ability to mark N/A.