If I'm understanding your question correctly, you want to know how to create multiple subjects and select from them when you open a case.
In CRM, navigate to Settings > Business Management. Click Subjects. Click Add a Subject. Enter the name of the new subject you want to create. You'll also need to select its parent subject, since subjects in Dynamics CRM are part of a hierarchy or tree. Finally, give a description and click OK. The screen may refresh, and then you can navigate through the hierarchy to find your new subject.
For more information about subjects, see the Resource Center article "Organize your data with subject trees".
--Matt