chmod to add local account to folder permissions...

Hoping someone can help. Normally I would use chmod +a group to add AD group permissions to existing folders on Isilon , and has always worked well.  Now that I am trying to do the same but with Local system account, chmod fails with illegal user argument.  Do I need to try the UID as follows ?chmod +a user allow dir_gen_all

I have tried chmod +a user "ISILON02\nasuser" ... But the "user"  argument is rejected. 

Anyone have feedback? Thanks