Creating plugin that will allow to batch add authors to po

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. By doing so, the author of the posts changes and the old author disappears . I want this change. I want that when mass editing the author of certain posts. the new author will be added, and not replace the old author. The plugin should change the appearance of the batch author change to be like the menu for batch selecting several categories - check boxes and not drop down. 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.