Is it possible to add mass floors in the conceptual mass family environment?
It does not seem to be possible - can't find a command to do it in the family editor.
Help files only refer to adding mass floors in the project (not the family).
Since you can add levels in the conceptual mass family, why not mass floors?

The reason for wanting to do this is that we have a building mass that is repeated many times in one project but each one sits at a slightly different height on a sloping site - we don't want to create hundreds of different levels in the project (one set for each building).
Linked files is a possible solution, but that has its own downsides.