Do you have existing NHibernate projects? Just point Visual NHibernate at the Visual Studio project and work with it visually. Visual NHibernate works with you, not against you, so you can stop using it at any time.
Visual NHibernate reverse-engineers your existing hbm config files to determine your mappings. Edit your mappings in Visual NHibernate - your hbm files will get updated with the new data. The source code of your existing classes will also get updated with relevant properties etc. without touching other existing code in the source-code file.
