Inserting shell script into .app bundle
This won't work for a signed app, but here's how I was able to get this to work:


Create a copy of the app you're trying to modify in case something goes wrong. (⌘D in Finder) For my example here, I modified the non-app store version of  VectorDesigner
In Terminal, cd /Applications/VectorDesigner.app/Contents/MacOS
mv VectorDesigner VectorDesigner\ copy (substitute the name of your app here
pico VectorDesigner (or your editor of choice) and add the contents of your script.

I used this script

 #!/bin/sh
 osascript -e "tell Application \"Finder\" to display alert \"Hello World\""
 /Applications/VectorDesigner.app/Contents/MacOS/VectorDesigner\ copy


The important thing is for the last line to call the renamed executable.
chmod a+x VectorDesigner


Now, when I double-click the VectorDesigner icon, I get the "Hello World" window, then it launches the original program.