Can you bleach a bit of head hair with body hair bleach?

DON'T do it.

Household bleach is meant to clean and disinfect things, not change the color of your hair. Household bleach is extremely caustic and will damage and burn your hair off. If you want your hair bleached you need to see a professional and have them do it for you. Stripping the color out of your hair is not a d.i.y. project that you do with products you have around your house, but if your looking for a cheaper alternative than going to a salon, you can go to a beauty supply store like Sally's, where you don't need a license to purchase products. They'll sell you a good bleach for your hair and give you further instructions on how to use it.