Hi - I'm attempting to use sip-conntrack-nat on my Slackware 12.0 box
with a stock kernel:

Linux version 2.6.21.5-smp (root@midas) (gcc version 4.1.2) #4 SMP Mon
Feb 11 17:13:56 CST 2008

I've read that as of 2.6.18, this module is included in the kernel. When
I attempt to load the module as instructed on
http://www.iptel.org/sipalg/ and elsewhere, I get an error that tells me FATAL: Error inserting nf_conntrack_sip
(/lib/modules/2.6.21.5-smp/kernel/net/netfilter/nf_conntrack_sip.ko):
Unknown symbol in module, or unknown parameter (see dmesg):

dmesg shows:
nf_conntrack_sip: Unknown parameter `ip_nat_sip'

Would it be reasonable to assume that although available in my kernel
version, that this module is not included in the stock Slackware kernel
that I'm running?

If so, is there a way to add this module without rebuilding the kernel?

Thanks for any hints!

That guide is old. The functionality is now included in the kernel. You do not need a kernel module to add the functionality.
In other words, you do not need to rebuild the kernel and you do not need to add a kernel module.

PS Welcome to LQ!