Increase artboard size by specific amount in Action / Script

I have something that I need to do frequently, which is to open a PDF in illustrator and increase the margins always by the same amount (5mm). I can do this simply by typing "+5mm" in the width and height boxes, but this doesn't seem to be able to be something recorded by actions, or that can be added into an action. Is there a way?

Failing that, is this something that can be easily scripted? I saw a few 'similar' scripts, but as it will always be 5mm and I'd prefer to run as a batch, I don't need any kind of a dialog box popping up, which also doesn't work with actions - so it would be better if it was a non-interactive script if it went that way.

Any help gratefully accepted at this point. A couple of fruitless (but kinda fun) hourse googling other possibilities have come to nothing. Maybe I'm not even looking in the right direction!