Any time you have to access NTFS security style with NFS/Linux clients, you need a valid Windows user that has access to the files/folders to map the UNIX user to.
So, yes, you would need to adjust usermap.cfg.
The mapping is only to help the filer determine *who* the UNIX user should be in Window land. After that, the ACLs control access.
Usermap syntax is located in the usermap file.