Web deployment projects lets you add output of any other Visual Studio project by using Add->Project Output dialog, where you can add Primary Output (your other project's DLLs, EXEs etc) and even Content Files (aspx, ascx etc). But what if you want to add external files, which are not part of any of your Visual Studio projects? You can add external files easily using Web Deployment Project's File System tab. These external files will be deployed on the target machine as a part of your setup project installation.
To get to the File Setup tab right click on your project in the solution explorer and select View - File System. On this page, right click on the folder that you want to add the file to (eg. Web Application Folder), in the context menu click Add - File and browse to the file that you want to add. You can add multiple files from any location on your machine.