How to provide a custom option for an exposed filter in Views?
if you are not able to achieve what you need with views default options.
You can create your custom views handler. In this case, it would be a filter handler.

https://api.drupal.org/api/views/handlers!views_handler_filter.inc/group/views_filter_handlers/7

https://drupal.org/node/1239866

Extending views functionallity to provide an approach to your problem