External library in Postman

No, linq.js, or any library not available in the Postman Sandbox cannot be used in Postman (by default, there is a workaround).

EDIT

Actually, if you fetch a script in a request and eval it, you can use it in Postman. An example is given in this blog post - http://blog.getpostman.com/2015/09/29/writing-a-behaviour-driven-api-testing-environment-within-postman/

I am doing pretty much the same than @grinderX19.

I run this once to load my global variable:

postman.setGlobalVariable("myUtils", function myUtils() {
let utils = {};

utils.function1= function function1(Arg1, Arg2){
    <code>
};

utils.function2= function function2(Arg1, Arg2){
    <code>
};

return utils;
} + '; myUtils();'
);

Then, I am calling it like this in Postman requests:

//import the global variable
let utils = eval(globals.myUtils);

//Call a function contained by this global variable
var var1 = utils.function1(arg1, arg2);

Hope this helps.