[Andreas]

Hmm, I don't know how I can describe this better

Do you have access to your DNS Configuration, either be directly editing BIND configuration or through a web frontend?
If you can't edit your DNS records in any way you're out of look, as this is absolutely necessary because otherwise nick.yourcommunity.com won't resolve to an IP address.

Furthermore you must be able to create a vHost for *.yourcommunity.com, again either by editing httpd.conf or through a web frontend.

When this is done, you can use smth. like

RewriteEngine On
RewriteCond %{HTTP_HOST} ^([^.]+)\.yourcommunity\.com$ [NC]
RewriteCond %1 !^www$ [NC]
RewriteRule (.*) http://www.yourcommunity.com/member.php?username=%1 [R,L]