You probably can, however it won't backup the data you put on the drive.
Let me explain ....
I use a 750 Western Digital MyBook ( Firewire ) for Time Machine.
In time Machine Preferences is shows drives that it cannot backup, or you can add drives you do not want backed up.
It won't backup itself and I cannot delete it from that list of " Do Not Backup " drives.
I have stored Data on there, but if the Time Machine drives dies, so does the data.
I have a 3 drive setup to insure data safety.
Internal Drive ( for OS / Applications )
External Drive ( iTunes / iPhoto Libraries )
External Drive ( Time Machine )
I could lose any one drive and recover, I could lose the first two and recover, but if I lost the Time Machine drive and one other, I could be in trouble!