[Edi]

Probably just asked. And probably after the fact too. If anyone does write the missing nation entries, Shrapnel might even host the errata/addendum PDF and put their official or unofficial approval on it. The people to ask would be Annette and Mindi. They are administrators here, so just send a PM and ask if you feel like doing it and want their approval.

So far as things generally go, the documentation I've done is all unofficial, but it's the most widely used stuff in modding circles. I expect that any manual addendum material, official or unofficial, would be widely used by those who like using the manual. I generally look at the manual maybe once every four months, so I have zero interest in putting effort toward the addendum.