You just have to create a shortcut where the target is the app plus the name of the file--like you would open the file from the command line.
For instance, "notepad.exe C:\test.txt".
Windows 7 will create a new taskbar item for every shortcut with a unique target address.
For most of the shortcuts on the Start Menu, all you need to do is create a copy and modify the target property on the copy. For some reason you can't do this for the shortcuts in the Accessories folder. I'm not exactly sure why, but all you need to do is create a new shortcut.