Our surveys do not allow additional questions to be added. Typically additional questions are asked during a follow-up with a customer after they have voiced their praises or concerns. This allows you to build a relationship with the client and gain additional feedback.
Even a handful (1-3) additional questions can cut your response rate in half. Plus most additional questions don't provide valuable insight into your customers. People will tell you what is truly important and driving their behavior if you let them. If you must or have a real need to ask other specific questions, hold these for a survey outside of NPS and try to send to specific customer segments who are interested or impacted by the topic at hand.