OK I managed to install on my existing /boot partition the kernel and the boot image of Slackware 14.0. I basically followed the instruction on this web site:
but without copying the whole dvd.
mount -o loop usbboot.img /mnt/tmp
mkdir -p /boot/slacklive
cp /mnt/tmp/* /boot/slacklive
Once I get that I edited my Lilo configuration to add the new boot option. I had to upgrade Lilo first to the Slack14 version because of some errors while writing the boot loader to disk.
The important part is to include the /boot/slacklive/initrd.img.