How can I add a custom field (stored as a field in Drupal db) under Manage
Events page ,which will be displayed as dropdown.
Event Status with values Pending or Approved.
How can I alter the filters added as seen in the image?
I would create replica of Manage event using Drupal views with same columns and filters as of Manage event screen.
If you still want to use the same screen then use pageRun hook to manipulate results by generating query and creating results and assigning to template
Use buildform hook provided by civi in order to add a custom input field on the form. Use this field as a filter eg Event Status.
From the value that is posted from this form(
$_POST['event_status']) - filter the result as mentioned by Pradeep using pageRun hook