Hi,
Actually I tried several month ago to create a local user and I had the same error. I totally forgotten this because i had to do something else. I thought this was a problem due to an upgrade on my OMV. If you encounteer the same error, I'm now thinking this is actually a bug.
I just created a new debian (7) server some days ago and it joined my domaiin with the exact same method I use for OMV. I tried to create a local user and it just worked.
I'll try to deploy a debian 6 and check the behavior of useradd. Stay tuned.