Hi!
As far as I know you are not able to do this directly with NHibernate, but you could implement a private property which composes and decomposes your fields to a string and map to that property instead of mapping your fields directly.
But I am not sure if this really something you should do in the first place. Usually requirements tend to come up during development and lifetime of an application and once you are going to need one of the fields - even if you now think you will never need to query for that field - you will have a hard time. Adding a column to a table of an existing database is not much of a deal and you still need to update the xml for every tuple in the table, so I really think it is better to store only one field in a column.
Best Regards,
Oliver Hanappi