You can't do that unless you have pre-compiled packages (kernel+ modules) coming from
somewhere (ie www.slackware.com
)., but I do not think they are available at the moment.
Consider kernel 2.6 series have different modules than 2.4 series and some different features as well (ie sysfs, usb2 support)
Slack 9.1 is fully compatible with 2.6 kernel series since it has the new module_init_tools already configured.
Compile the new kernel per "readme file" using the /boot/config file as guide.
It won't be difficult at all. You'll need only some small adjustments to fine tune yor box.