This is by design. The Customer Center, Employee Center, Partner Center and Vendor Center cannot be selected as Center Type for roles created from scratch. These centers have specific permissions built in to the role. Customization is only limited to changing the access level.
If a user wants to create new roles based on the above centers, that has to be done by customizing the Standard Customer, Vendor, Partner and Employee Center roles.
Steps to Reproduce:
1. Log in as Administrator.
2. Navigate to Setup > Users/Roles > Manage Roles.
3. Click New.
4. Enter Name for the role.
5. Click on Center Type dropdown.
Customer Center, Employee Center, Partner Center and Vendor Center types are not available.