You can define your functions in the query.substitutions part of hibernate configuration.
Code:
<property name="hibernate.query.substitutions">true 1, false 0, month MONTH, DAY DAY, getDate GETDATE</property>
the code above should instruct nhibernate to replace any occurences of the word on left with the word on right.
What I'm not sure of is if the getdate function with no parameters will work, but remember is only available for single parameter functions.
Hope it helps.
PS:
http://www.hibernate.org/hib_docs/reference/en/html/queryhql.html