Because the igb kernel module is also in the main kernel, you're probably going to have issues. "module igb" in build.conf could grab either one. So, you could make it as an extra kernel module as Don suggested, but you might need to tell it to rename itself to something other than igb.ko. I don't know if that's an option or not in the module's makefile.
Is this possibly better solved by jumping to a newer patch of the current long-term stable kernel in Thinstation? Just change the VERSION under kernel-modules and then rebuild the kernels and modules under there (I think there's a script for that).
As long as the module is not installed in the same place as the in-tree kernel driver, you could just call repackage on the port to make a package by the same name. Or for a hack, just make a package with the module in the right spot. Just make sure you don't include both igb modules.