The wizard in #1 (Data Model > Classes & Properties > Database Class Mappings ... New External Database Table Class Mapping) is a faster configuration if the database table already exists, as it will list the columns on the table and propose property names which you can edit to have them meet Pega naming conventions in use on your project. 

For #2, you would have to first create the properties on the new Class manually, then configure the mappings on the Class rule for any property that does not exactly match the database column name, and as you mention, add the DB Table instance from SysAdmin.