Changing the Create a plugin that will allow to batch add se

My site is avelim.co.il. The site is on Hebrew wordpress 3.3.2, using the theme Weaver 2.2.6 with many customization done by programmers. Currently in wordpress,you can batch edit posts and change the author of several posts at once. You can not select more then one author doing so. What I want, I want to be able to add several authors to a batch of posts post, using boxes. just like you can assign several categories to a batch of posts. For this, I need a custom plugin built. The freelancer should send me the plugin as a zip file which I will install on test site. When the plugin is working properly, I will install it on a live site. The work should be done in 24 hours after being awarded the project. Freelancer should allow 7 days for testing after he finishes his work, before getting paid.