There is a thread
in the Slackware forum that describes how to compile your own kernel. Also, if you do a search here you will find loads of threads on the subject. But basically, you'll need to download and/or install the kernel source code. If you don't want to change your kernel version, be sure that you get the source code for your current kernel (uname -a should identify what version is currently installed). Once you've got the source installed, you'll need to configure it. That is done with either make menuconfig (if you're running from a console) or make xconfig (if you are running X). There are loads of options to set, but there should be a section on disk format support and you'll find the NTFS support in there.
Kernel compiling seems scary at first, but once you've done it, you won't be able to shake the addiction!