Without knowing what 'Mac OSX Journaled' is, I don't think this is gonna work.
First, the partition are not 'flat' partitions. There are raid arrays and logical volumes. (More info here
Second, the box doesn't share a filesystem, it shares files, just like ftp (except for iSCSI, if the box supports that). That means the client cannot know how and where the files are actually stored, and the server has to mount the filesystem. So if you put an exotic filesystem on the data partition, the nas won't recognize it.