A 7-mode file server likely does not need to be maintained for restores. The 7-mode and C-mode backup formats are compatible. The volume naming is different between 7-mode and C-mode so it will not be possible to do in-place restores, but out-of-place restores will work. For example if you have a 7-mode backup of "/vol/businessoperations" and you want to restore it to C-mode "/businessvserver1/operations" then browse and select "/vol/businessoperations" to be restored, select the destination file server client to be restored to, set the destination path to "/businessvserver1/operations" and set preserve level to 0. A similar procedure can be used to restore subdirectories and individual files.
As Winston mentioned, you can engage Support and we can look at your specific configuration.
PS: Note that if the NDMP backup was done from a backupset which has the "Image Backup Set" option selected, then the 7-mode backup cannot be restored to C-mode. This option is not typically selected - it is used to run a NetApp "smtape" backup which is a block based backup of the file server volume. When this smtape backup is used, only whole volume backups and whole volume restores are possible. And the browse of the backup data will only show the volume itself - the browse will not show the contents of the volume.