Yeah, in slackware you won't have an automated partition creation tool, but its easy enough to do it manually.
When you login as root, type "cfdisk /dev/hda" (replace hda with hdb for second IDE drive etc.), and then select the free space in the drive. Create a new partition, and assign it the type "Linux". Then write the partition table, exit cfdisk and type "setup" to begin.
You can also use your Partition Magic to create an ext2 fs, and then the setup tool of slackware will format it again as the fs format of your choice.
BTW, its not recommended to use ext3 filesystem here, slackware uses reiserfs by default, faster and better, but if you have an old system, you can use ext2 if you don't want the features of journalled filesystems (basically it reduces the risk of data loss is system isn't shut down, by maintaining a journal of disk activities).