No. Punching a bag is not good warm-up for wrist extension.
I agree that it sounds like you don't get where the weight of the bar is. It's resting on your deltoids, crossing in front your clavicles, and hands don't hold the weight. They tell me that you get used to the grip with persistent practice. That hasn't been my experience, but I'm old, and probably haven't tried hard enough to stick with FS.
Were you making a joke about CrossFit? It has nothing to do with "cross grip".
Ask or answer questions, discuss and express your views