Can add_image_size be added earlier

You can set the priority to 999 to make sure if you are going that way:

add_action( 'after_setup_theme', '_my_action_add_image_sizes', 999 );

but if you really want to make sure of that order, try making a custom action, here a small example:

/**
 * Execute the action with do_action.
 */
 function read_theme_options() {
   //all the logic you want to execute before adding image sizes here

   do_action( 'my_action' );//do the action
 }

 /**
  * Register the action, you can add the functions you want to the action
  */
  add_action( 'my_action', '_my_action_add_image_sizes' );
  function _my_action_add_image_sizes() {
    //adding images here
  }