I don’t think it’s possible to have an independent object not resting on the build plate. Such an object could never be printed, so the slicer should not allow it.
The one time I needed to do this, I made a “kickstand”, a really small/thin post the object sat on top of. Disappeared when sliced, but the slicer obeyed the geometry and kept it suspended in the air like I wanted.
But I think if you load multiple objects as a “single object”, the slicer will respect the relative positions of each object relative to a common origin point. So if one part is 1" above the other, they’ll load that way and the slicer will generate supports to reach the higher part.