Hi Grace,
Floor Manager creates a series of reference planes that set the active Auxiliary Coordinate System (ACS) once selected. They are always horizontal.
Assuming that the sloping floors involved are flat planes it would be possible to create an ACS oriented to each plane, but these cannot be managed through Floor Manager.
Please take a look at the Create ACS (By Face and (By Points) tools.
These tools can be used on-the-fly for local modelling operations or if a consistent set is required for sharing you could create a set of appropriately labelled ACSs in a shared file available to your team. The ACSs could then be imported (via the ACS dialog > Import ACS button) when required into design models. There is a time-saving key-in: 'acs import', that goes straight to the import dialog, skipping the ACS dialog.
If this is a helpful workflow it would probably be possible to automate the import of particular ACS using VBA.
Alternatively the ACSs could be added to project seed files.
Note that the ACSs are not connected to any elements placed using them, they are simply reference planes used at placement.