To use tape drives accessed through the floppy controller, you need to install the proper kernel driver. Of course, this is generally true for all drives. First of all, you need to recompile your kernel and enable support for QIC-117 ( FTAPE ) (and, of course, install your new kernel). Before using the new driver you not only have to reboot but also load the module into the kernel. A simple recompilation won't do. Then, KBACKUP should work with your device.
KBACKUP does not work with zftape .