I used to use a little ap called numlockx that'll set your numlock on. You could put it, say, in /etc/rc.d/rc.local. You can probably still find it and it's a simple tarball that'll make an easy slackpack with checkinstall.
That'd be fine for runlevel 3, but if you boot into runlevel 4 numlockx is killed by the new xsession and has to be started again.
That don't matter much if you're into kde since you can go to Control Panel, Periphials, Keyboard and set numlock on when your kde desktop opens up.