Well, after reading the question for the nth time with this exact problem, I decided to write an implementation that does not include an SQL record.
You can check the implementation at http://savale.blogspot.com/2011/04/nhibernate-and-missing.html , with which you can write:
criteria.Add(
Restrictions
.GeProperty("Prop1",
new ArithmeticOperatorProjection("+",
NHibernateUtil.Int32,
Projections.Property("Prop2"), Projections.Property("Prop3")
)
)
);
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