That a second set of Z-liner pads would fit in the pockets is consistent with a "suggestion" in another thread that DIY climashield pads could be thicker and warmer, or that the fill could and should be down.
I wonder how nx-250 z-liners would fit as 2nd layer for my nx-150? I know...ask Clark
for very cold excursions, I use my Zliner in addition to a down UQ. Works nicely, but bulky, so not for backpacking so much.