Oh, yeah, that's actually not really documented but it's pretty straight forward:
Rebuilding the ramdisk with the firmware added. To be honest i'm not 100% sure anymore if i had to also add some module but it would be basically the same procedure of editing the file list for the ramdisk and running make from the ramdisk directory. Make needs a couple parameters to work outside of build.sh (from the top of my head mostly setting building toolchain to never so the local toolchain gets used and strangely enough cflags containing -m64 as it tried building 32 bit binaries even though my system is running amd64), one or two dependencies sadly need small one line hacks if i remember correctly and one dependency needs to be built before the ramdisk is so make doesn't error out because of a missing file (not sure why it isn't built automatically) but in general most people somewhat proficient with the build process should manage to do it (i have hardly anything i'd call experience with NetBSD).
I'm very sorry if that isn't as detailed or exact as it could be but i didn't really take notes during the process as i had planned to do so at a later time while going through the whole process again on a VM. I'm absolutely going to expand the howto (it seems to be somewhat broken with 10.0 anyways - i probably had more of a hard time getting a bootable system going than making iwlwifi work) once i have to the time to do another install but i'm sadly kind of busy at the moment and i don't know when i'll get to it yet.