Can a Catholic be excommunicated for willful disregard of Church law?

This relates to a discussion on another Question (which I won't link because I don't want to draw attention to the OP). The following is all hypothetical. (I, personally, am not Roman Catholic, nor do I have any plans of becoming so.) Let's say I engage in some practice which I believe to be beneficial to my health. (The specific practice is irrelevant.) Let's say, also, that the RCC forbids this practice. However, I believe that my health is more important than adhering to Catholic teaching, so I continue the practice in genuine belief that doing so is critical to my health and longevity. Could I be excommunicated for this? Does it make a difference if I was already Catholic, vs. if I converted with the intention of ignoring the RCC prohibition to refrain from this practice?