FS700 handle has Arri rosette interface where as FS100 is a simple screw-on. Physically, it's possible. Both uses the same size screw so you can just screw on the FS700 handle to FS100. The down side is it wouldn't be very secure and not functional at all. Also the metal rosette would completely scratch up the body -- you can put a thin washer in between to prevent the scratches.
I installed fs700 handle on my FS100 with simple and light Berkeysystem rig. all buttons make their job exept photo button. I bought the fs700 handle 220€ HT
So, i have expand focus rec and auto iris buttons on the same handle.
The rig is well balanced to my sigma 50-150mm and tokina 11-16mm and of course sony18-200mm.
I'm waiting a base plate from berkeysystem for plug and unplug easily my rig on tripod.
I hope that will help some people.