I have a feeling this is very easy to do. I am not familiar with any acrobat scripting. Where's the object model viewer? How do you add the script?(I hate this question)
I know I can figure it out with a little research. Any help would be a very appreciated.
Here's a link to the documentation. You'll want to use the setPageBoxes method of the Doc object, after using getPageBoxes to get the crop rect of the page.