Well post WP 2.5 how categories are stored has changed. Now its stored under term_taxonomy.
From what I understand you want a group of posts to be added to another category. So after you select you will need to then update specific term values by joining tables.
If the number of entries is less - I guess its better you simply go to posts and modify / update the category by selecting multiple posts and at once running update.
If you still wish to go via the database route - these posts might help you