The only way is to participate in one of our armatar-contests. Of course the entrys must be appropriate and fit to the theme of the contest.
Check this one for example.
All acceptable entrys are choosable for a limited time after the contest. Users who choose one of them hold the choosen one as long as they not change it, after a change they cant get it again (if its not in the roster again).
Besides this "participate-in-our-seasonal-armatars-contest" option: no, users cant make/choose custom armatars
To continue on @UnleashedUponMankind 's answer to your question... some of the Armatars you're seeing might be from previously available batches of Armatars, which are not available anymore. They might be released again, but this is never certain.
Also, I'm moving this to the Support Forum, as it is better suited there!
