Fragging in the tank can be difficult. When it's my only option I use a sharp exacto knife and try and cut some of the live rock with the zoas (meaning I don't just cut the zoa tissue off the rock, but I that i shave off the top potion of live rock the zoas are attached too). IF you have patience you could always set a piece of rubble rock or a frag plug next to your zoas and let them grow onto it.
Oh and if you frag in the tank, make sure to turn all your pumps off. I've lost zoas when I forgot to do this.