In the docs:
files (Optional) Drupal supports a dynamic-loading code registry. To support it, all modules must declare any code files containing class or interface declarations in the .info file, like so:
name = Really Neat Widget ... files = tests/example.test
When a module is enabled, Drupal will rescan all declared files and index all the classes and interfaces that it finds. Classes will be loaded automatically by PHP when they are first accessed.
But, it is posible to add a file fomr another module?