If it is only jQuery then I would say take a look at the AppStoreBoilerPlate widget from Mendix on GitHub. It is built with an AMD version of jQuery loaded into the Main JS file.
It's straightforward to convert your existing js library to an AMD-friendly format. I have followed and recommended this article to people before, and I think it lays out the steps well.
Once your module is AMD-friendly, you can load it just like any other module
One option could be to modify the non-amd library to attach the widget to the window. eg:
var myNiceModule = someReferenceToYourLibrary
//This is inside that library
window.myNiceModule = myNiceModule
But if its just the jQuery library you are struggling with, get the version from the app store boilerplate.