ICriteria Restriction on Expression
DetachedCriteria criteria = DetachedCriteria.For<Order>()
    .Add(Restrictions.Eq("Property1 + Property2", confirmation.Ammount));

.Add(Expression.Sql("(Property1 + Property2) = ?", confirmation.Ammount, NHibernateUtil.Int32));

.Add(Restrictions.Eq(new ArithmeticOperatorProjection(
    "+", NHibernateUtil.Int32, Projections.Property("Property1"), Projections.Property("Property2")
    )
)