Backing Up OS X Server to Local Time Machine Server Folder
No, you cannot. Backing up to a local volume (what you're having the server do) uses a slightly different backup format than a network backup (what your server is allowing clients to do). With a local volume backup, the backups are stored inside a folder named Backups.backupdb which is at the root of the backup volume (not in a subfolder). With a network backup, the backup is stored inside a disk image, and the Backups.backupdb folder is at the root of the volume inside that. Because the network backup is wrapped in a disk image, that image can be almost anywhere (volume root, folder, subfolder, whatever); but this flexibility does not exist for a local volume backup.