Kelby,
If you are cautious and risk averse then put a shellac coat on the Z-poxy before applying the nitro. This is on the basis that pretty much everything sticks to shellac - the shellac sticks to the Z-poxy and the nitro to the shellac.
If you like risks then go with the nitro directly onto the Z-poxy. It's going to depend on the nitro you are using but there could (I say could) be adhesion issues, and just to be interesting (as most things in guitar making are
) these adhesion issues may not surface for a year or so down the line.
Best bet is to check with the manufacturer of the nitro.
I have read in various places that you should never put shellac under Z-poxy as the Z-poxy doesn't stick to shellac but shellac will stick to Z-poxy that is on wood (interesting one). I have never tested this out so can't confirm or deny this one.
That would be (and is) my choice.