Hook name in module doesn't seem to work
there is no hook_buildForm() hook in Drupal. The hook you are trying to implement is hook_civicrm_buildForm(), so to get it work you should use mymodule_civicrm_buildForm().
You can always use hook_form_alter() or, more specifically, hook_form_FORM_ID_alter() to make changes on any Drupal form.