There isn't a way to assign a script to multiple or all pages. This will be a useful feature, Your question ties to some other threads in the forum, around Axure applying object orient approach - in this case - the ability to create scripts in a snippet fashion and apply those to objects (pages, widgets and so on). Although, the current method, while labor intenssive is a straight forward method to ensure the proper context of each script.
Best,
-Ezra
Disclaimer: I am not an employee of Axure nor am I compensated by the company in any way, shape or form. Rather, I have a vested interest In its continued development as an avid user of the application on a daily-basis. (disclaimer by Ari Feldman)