I do not understand your problem to add new maps.. all you have to do is to download them, put the pk3 files into Nexuiz/data and add the right mapnames to g_maplist in you server.cfg.
I really had hoped that good enough explained in Nexuiz/Docs/server and also in this thread here in the forum: http://forums.alientrap.local/viewtopic.php?t=18
(Nexuiz/Docs/server/readme.txt should contain that link too)