Adding more fields in the sign-up panel

Dear all;

How are you?
I would like to add more fields (questions) in the sign-up panel (see the attached) such as "what kinds of classes have you taken before?". I am wondering if I can do it. If so, could you explain how to do it?

I tried to add a new field in signup/theme/signup_form.inc. But, it seems not to work.
If you have some documentations how to do it, please let me know...
I would like to query it later using views, too!

Thanks for your help in advance. : )