If you can access the database directly, you could
- Check for the ID of the category you want to assign
- Check for the Author ID
Then run the following MySql (on your own risk):
INSERT INTO `#__term_relationships` ( object_id, term_taxonomy_id )
SELECT ID, 1 FROM `#__posts` WHERE `post_type` = 'post' // where 1 is your desired category ID
UPDATE `#__posts` SET `post_author` = 2 // desired author ID