Check out the following discussion on the WeBWorK wiki.
There is a CSV-like file format that you can use to export a spreadsheet of information about students and it automatically creates the corresponding students when you import the .lst file into the course.
Be sure to note from that discussion that the password is set to use the student_id unless you take extra time to generate crypted forms of other assigned passwords. Students can (and probably should) change the password after they login.
If you need more than this, feel free to ask a follow-up.
- Brian Walton