bigswp on "Can set_post_thumbnail be used like this"

I have a folder in:
wp-content/uploads/sites/2/thumbs/ (this is a multisite)
that is being used by a plugin to display images on the front page. I would also like to use these same images as "Featured Images" can I somehow use:
<?php set_post_thumbnail( $post, $thumbnail_id ); ?>
for this purpose?

I should also point out that the images are already using the post id as their name... example: 3301_1.jpg (3301 is the post ID)

It seems to me this would be a lot easier than trying to go in and set all these manually.