First of all, if you are a prof in the admin course, you will by default be added as a prof to any new course that is created (unless whoever creates the course unchecks that option). Of course, a prof in that course can remove you.
If a course has already exists, your only options are (1) to have a current prof in the course add you or (2) if you have access to the server, look at http://webwork.maa.org/wiki/WeBWorK_shell_-_wwsh
Hope this helps.
Arnie