Yes, but you might not want to.

There are 2 ways to do this that I’m aware of:

1. Put your code (or some of it) in your jsextensions folder.

2. Use File + eval:

/* seriously, don't use this unless you really know what's in the file. */
jsInclude.local = 1;
function jsInclude(filename) {
    var f = new File(filename);
    eval(readWholeFile(f));
}

readWholeFile.local = 1;
function readWholeFile(f) {
    var BLOCK_SIZE = 1024;
    if(f.isopen) {
	var s = "";
	var l;
	while(l = f.readstring(1024)) {
	    s += l;
	}
	return s;
    }
}

I should mention, there is a third way, which is to use standard #include directives and then run your code through the C preprocessor. In other words, write your code in separate files, and use #include "myothercode.js" where you want to refer to another file. Then run the central file through the preprocessor to produce the file you actually point max at.