What you want is a chroot environment -- that means you start with another from environment, for example from your Slackware CD and then "chroot" to your Slack that's installed on your harddrive.
To do this boot from your slackware CD. When you are at the console and you get the message about cfdisk and setup, stop. Now you have to make some directories in the tree. Let's suppose you do this stuff in the /mnt dir. What partitions do you have? If you have a seperate /var directory on your harddrive install you have to mount that, too.
I hope there are no errors in my code, but that's the way I did it.
# do this step for /var if you have seperated it on an own partition
# now mount it and chroot
# insert your root partition and partition type instead of /dev/hda5
# and reiserfs
mount -t reiserfs /dev/hda5 /mnt/slackroot
# install the package glibc or others as well
# if Slackware 10.1 and if your cdrom is mounted at /mnt/cdrom
# now let's switch back to cdrom, else you cannot shutdown properly
# shutdown -- be sure to remove CD-ROM before booting again
shutdown -h now
Edit: Rewrapped lines | added .tgz ending to package name | fixed typo