Is it possible to change query which is being generated for custom type/mapping?

Like I have 2 date fields which work as a partial date structure, sql query generated by NHibernate is not valid.

It generates:

DATE_FROM >= p0 AND DATE_TO >= p1

I need:

DATE_FROM <= p1 AND DATE_TO >= p0

Another option is to use string and process it with custom query.
Yet another option is to create custom user type in database (not sure whether NHibernate supports this).

But I think I would have to handle this in the .Net code or avoid using NHibernate for this entity. :(