I have installed an app into sanboxie but i now want to add a file into the applications program files folder how can i do that?

Open a Windows Explorer instance, go to the folder where the file you want to copy is located, and copy it. Then right-click Sandboxie´s tray icon and click:

DefaultBox (or the name of the sandbox) > Explorer contents

Another Windows Explorer instance will be opened, but this time the path will be located where the sandbox is. Just navigate through folders (something like "drive > c > Programs folder > ...) and paste the file in the right place.