The position of Z is defined in the import script by the value you use as the Z scale × the value of the heightmap value.
You really should not have to do anything special even across multiple imports if you keep the same scale values and the tiles come from the same larger hightmap.
If you need to shift a level up or down you just have to do it correctly so that world composition knows its location.
You can do so within the Level panel. And at least as of 25.1 the changes are kept on reload.
beware. Shifting the level this way makes it so that the heightmap will no longer dovetail to the adjacent one.