Qinoa

Question

Regarding the issue of how Quinoa can be considered kitniyot, didn't R. Moshe Feinstien Z"lexpress the opinion that Kitniyos is not a Halacha (official law) but a minhag (custom), and that while minhagim often have the force of Halacha, a minhag cannot be extended beyond what was actually included in the custom? (For example, in Igros Moshe O.D. III:63 regarding peanuts, doesn't R. Feinstein say that if a food was not in common use in Europe when the minhag of Kitniyos was instituted, there is no Halachic basis to extend it to such items. Kal Vechomer, wouldn't such logic apply to Quinoa? Respecfully, Barry Goldberg