SI gave you one good way to put hair in.
octoberist gave you a second option but make sure you use clear silicone not the stuff with caulk maxed in it.
and here is a third way: I use alot of Dragon Skin to make my masks which is a silicone based product. and I to use punching and wig hookng to put hair on.
I buy the small screen at Lowes or HD that you put in Living room windows and i cut as much of the screen off as I need.and I hook the hair into it like a wig net.Then use clear silicone to put it on my prop heads.but first I take a dab of silicone and rub the back of the screen to lock the hair in. I let it set up then I silicone it to the head.This way it does not pull out. after it has completely set up I then go back and give the prop head a hair cut.Hope this has helped and you get a more natural look this way.
Here is some of my work All these head were made from the same mold.