Ldap shadow expire

[decor at supagro.inra.fr]

hi,
In a ldap authentication i try to check the expiration date.
With default ldap schema there is ldap attribute named "shadowexpire". And
i'm pretty sure the format is unix timestamp.
Can i use it directly with freeradius ldap.attrmap like that :

checkItem       Expiration              shadowexpire



???
i've found an answer saying : you must create in ldap a new attribute named
radiusExpiration and check it in ldap.attrmap. why don't use directly
shadowexpire ?

thanks