In qualitative interviewing, whenever you yourself lack the expertise to write up an interview guide, then an expert or "key informant" interview can help you. It can also be helpful when you are unsure about to recruit participants, or to establish rapport during the interviews.

I am not such what you mean by a "quantitative expert interview."

You might be able to justify expert interviews as part of a case study methodology. I describe this in chapter 3 methodology of the attached, although interviews were only incidental to the research approach.

Thesis An analytical tool to aid the reflective selection of equity...

Oxford professor @Bent Flyvbjerg is the leading authority on case study methodology and he is also active here on Researchgate.

https://www.researchgate.net/profile/Bent-Flyvbjerg

thank you