It's possible to request your students to fill in their student number even after they have already created their account. To enable this option, go to your class Settings-Student data. Enable: Students have the option to input their student number.

This is the pop-up that the students will see the next time they go to your class on SOWISO:

Students can click away and still not fill in the student number. It's not possible to force them to fill this in.