i'm assuming you're going to unscrew the detonator head, plug the hole that they drill in the bottom of every shell, and some how grind the threaded well for the detonator head down and plug that up.
its possible but it won't be easy. I've owned that same grenade before (M67 frag nade. the one in the picture there is a practice one though so its constructed a bit different), that shell is steel alloy and tungsten if i remember. unless you have access to some good power tools and some replacement heads, it's going to be a long and arduous job to turn that into a plasma grenade.
alternatively, with a bit of bondo, you could add some more details into it and modify the soon and make it into the human frag.