you find settings for this under
ACP - tab GENERAL - user registration settings
right side: general settings
"Limit username chars:
Restrict type of characters that may be used in usernames, spacers are: space, -, +, _, [ and ]."
There you can choose whatever you want.
oh, david was faster...
Webseite, Blog, Wiki
Deutsche Übersetzungen - german language files
My 2 cents: Whether an extension is in the CDB says nothing about its quality. It is more important to read the support topics for it. Better to avoid authors who do not answer support questions themselves, who do not update their stuff, and who do not fix bugs for years.