Is it possible to put Drupal.settings javascript in separate file
Yes - this won't make a difference to Drupal where the file is coming from. In Drupal, you can load this using drupal_add_js:

drupal_add_js('http://example.com/example.js', 'external');


Note that these files will not get aggregated when preprocessing is on.

The other thing to watch for with external javascript is to make sure you are testing for the existence of functions / variables if you are calling them in your file since you can't control the order / speed that they are loading in.