Comment 4 for bug 1112306

I did look up the code and there are three occurrences where usermod is called.

check.py
base/queues.py
installer/core_install.py

check.py looks up the information in installer/distros.dat and if nothing is there it prints a log message.

installer/core_install.py looks up the information in installer/distros.dat and if nothing is there nothing happens.

base/queues.py which I think is responsible for the issue looks up the information in installer/distros.dat and if nothing is there it executes:
usermod -Glp username

So the "-a" is missing here. Your patch should fix the issue for Gentoo, but I think the queues.py should be fixed as well. From looking up installer/distros.dat Suse seems to be the only distribution which does not add the "-a" flag. So leaving it out in the general case seems wrong.