NHibernate conditional computed field (Hibernate can help too)

Question

I need to create computed fields in NHibernate, but in these fields I must use conditional expressions. MySQL example:

if(now() < addtime(?1, ?2), 'true', 'false')

The computed field is stored in the database, and when the program needs this field, the program gets it from database, uses it to create a projection and finally, NHibernate proceeds with the execution of the full query.

How do I create any computed field containing functions with NHibernate instead of creating the field manually for each dialect (Microsoft SQL, Postgresql or MySQL)?