Add a strip of "water" at exactly y = 10

I am trying to make an inverted dimension, where land is water, and water is land. i found the place to do this(in the final density section). But now i am running into an issue, where there is no floor as a barrier to the void. 
I am wondering if, given my file, there is a way to add a plane of water, that will be inverted to stone, that stretches over all of the area below y=5? Preferably without changing the surface.