AppleScript to add files to an iOS app under iTunes
As far as I can see in iTunes' AppleScript dictionary, there's no support for scripting anything relating to iOS applications. Sorry.

A possible workaround:

If the app, and your Mac, support iCloud, set it up on both and look inside "~/Library/Mobile Documents/" on your Mac.

In there should be a folder corresponding to each app you have iCloud enabled for. You might have to look around the sub-folder structure to see exactly where the files you want to add need to go, but once you figure that out, you can write your script to target that location.

When your script adds a file to that location, it will sync via iCloud. I use this trick all the time to get files into my GoodReader app.

Beware that iOS apps don't always sync to iCloud in the background. In the case of GoodReader I need to open it for the changes to update.