I think the short answer is that there isn't a good way to do this in general. Bill Wheeler's LTI package allows students to be auto-added to a WeBWorK course when they are enrolled in an external course database, but that's a step more complicated than what I think you're discussing. Note that students can edit their e-mail addresses once they log in, so a possibility if you know who the students in your course are is to add them with blank e-mail addresses and then ask them to update that information.
Gavin