So, instead of sewing pieces of popcorn onto a knit hat, you want to make a hat shaped like a giant piece of popcorn? Hmm, W&N seems like the way to go. I'd make a tube of it, bigger than the circumference of your head, stitching one end closed. Then I'd poke bits of batting scraps or fiberfill into pockets, and tie those shut with either thread or white rubber bands, creating knobs around the tube. W&N will stretch a bit, and that will help. Once you are done, you can stitch some elastic around the open end to make it fit your head.
I dunno how clear that is to you. I'd just play around with it, and see how it goes.
Personally, I'd sew popped corn to a knit hat! It's not hard to sew - I've strung lots of garland with popcorn for Christmas trees, before we figured out our dog loved popcorn, and would go to any lengths to get it. Then came kids, same problem! Someday, I'll do it again - I love the look and it's fun to do!