Is it possible to dynamically add mapping files to the SessionFactory? We are building a generic runtime system that will dynamically generate in-memory mapping files from databasee schema metadata and would like to lazily add these generated mapping files to the SessionFactory at runtime.

No, not without much hackery inside SessionFactoryImpl. However, there's nothing to stop you from discarding your old factory and creating a new one. It's not a speedy operation, not something that you'd do for every request, but presumably adding new mapping files would be a rare event anyway.