Comment 24 for bug 23475

It seems there is a standard problem:
$ sudo adduser håkan
adduser: To avoid problems, the username should consist only of
letters, digits, underscores, periods, at signs and dashes, and not start with
a dash (as defined by IEEE Std 1003.1-2001). For compatibility with Samba
machine accounts $ is also supported at the end of the username

http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap06.html#tag_06_01
http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html
Alias Name:
"In the shell command language, a word consisting solely of underscores, digits, and alphabetics from the portable character set and any of the following characters: '!', '%', ',', '@'.
Implementations may allow other characters within alias names as an extension. "
and Name:
"In the shell command language, a word consisting solely of underscores, digits, and alphabetics from the portable character set. The first character of a name is not a digit. "

On the other hand, I've used the useradd command instead of adduser:
sudo useradd -m håkan

and the user got listed in /etc/passwd:
håkan:x:1004:1004::/home/håkan:/bin/sh

I could login and use it, but I wonder if it will break any programs..