You'll have to know your slack kernel name before doing this, if it is /vmlinuz on hdb1 just add to menu.lst
title = Slackware
Be sure not to install another instance of grub on hdb1, since grub searches for menu.lst at boot time, it will use the first one it finds, and this may eventually drive you crazy
PS: Having swap partition on a different disk will probably even increase performance (if you use swap a lot) if the disks are on different buses (e.g. hda and hdc) since disk head doesn't need to go back and forth.