W2K Security for File system repository without user mapping?

Hi,

Is it possible to set up a file system repository under EP5 SP6 that uses W2K security (windows file permissions are mapped through) without every user maintaining a user mapping for the Microsoft_Windows_KM system?

The portal is setup using NTLM and all user authentication is against MS Active Directory. I don't understand why a user mapping would be requried?

Thanks,

Ben