As far as I know, you can't do it so it all works automatically. You can, however, add the data onto the end and then pull it off when you want.
To do this (and I might get this very slightly wrong) :
- Take the mksysb as normal
- Using tape device rmt0.1 (no rewind) fast forward through the tape to the end of the mksysb. (I think this is tctl -f /dev/rmt0.1 fsf 4)
- Take the backup