How to answer
If you have never worked with steel plumbing, you can certainly let the interviewer know that you haven't but be sure to talk a bit about what you do know.
"I have personally never used stainless steel plumbing underneath a sink. If I were to, it would be in instances where it is more cosmetic. For example, on a clawfoot tub or wall mounted sink. What I do know about steel plumbing is that it is chrome plated brass in order to avoid corrosion."