Help with moving jaw

(I hope I posted this right, I'm new here ^^; ) This might be a dumb question but I was wondering if it is possible to make a moving jaw on my already foamed fursuit head? As you can see, it's a balaclava however it is already foamed(I bought this from someone that way). I've read tutorials about sewing/gluing elastic on the balaclava but couldn't find any about already foamed heads.

Thanks!