Implement a custom PHP library into a custom block module?
drupal_add_library() is for Javascript/CSS libraries; Drupal doesn't have a similar registry for PHP libraries/classes.

To include a file inside a module in your installation, use module_load_include() e.g.

module_load_include('inc', 'mymodule', 'mymodule.pages');


For all others just defer to the standard PHP functions (include/require, etc.)