"Can you use the FTZ adapter on the new Z teleconverters?"
No.
Nikon didn't supply a reason, but it's possible that the baffles at the back of the FTZ adapter would interfere with the extending elements the protrude from the front of the TCs. But I suspect the real answer has to do with the position of the optics. Once the TCs show up next month, I'll take a close look to see what the issue is.