local Admin changing to built in Admin during WFS migration

On a WFS ENV(Windows file server) there Local administrator group which has admin rights on the source . When we migrate with all the permission from source to target this Group is no longer part of the isilon ENV and is replaced by Builtin/Administrators group on the isilon . Only 3 users are part of this group. Due to which we cannot view or modify any files on the target . We need the administrators group to have access on the isilon for the admin to have control and modify the files. we have tried adding the local admins directly to the built in admin group but still was unable to access the any folders we have group on the target which is part of the ad but when we migrate those groups are removed and the source group are added so now under the present condition we have manually add the group to the folders or people individually to this group and if we assign users directly to the folders on the target that will take alot time when we have 1000 files on the target. Does anyone have ideas on how to bypass Isilon changing the local admin  to built in admin from source to target?