You can install a kernel to slot A, B or *NWSSTG by executing the following command:
dd if=<your kernel here> of=/proc/iSeries/mf/A/vmlinux bs=4096
Note that you can change "A" to "B" in the previous command to install to the B slot and if you want to install to *NWSSTG you would change the following "of=/dev/hdaX" where X is the partition number of your PReP boot partition.
You can also put command line parameters into slot A and B using:
echo "your command line here" > /proc/iSeries/mf/<A or B>/cmdline