This may sound counterintuitive, but to turn your passives into promoters, it will actually be very useful to find out what your promoters are saying. To get useful feedback from this group, word your open question carefully… ask very specifically in your Net Promoter Score survey if they had to choose one reason why they are so satisfied, what would it be. With a key driver analysis, you will be able to identify which aspects of your service or product influence how likely a customer will be to recommend you to others.
Once you know what experiences are leading promoters to be so satisfied, put effort into replicating these experiences with more customers which will lead to more promoters and more growth.
Another positive aspect of this approach is that employees often experience NPS programs as something negative, since most of the information that drips down to them are issues that detractors are having. By also focusing on the positive points brought up by promoters, you can set up a positive feedback loop among employees and improve their view of your NPS program.