Multi-File Uploader: add file via JS?

I’m trying to add a file to the multi-file uploader field via JS. I’ve got an image URL and I’m loading the data in. Then I want to add it to one of my form’s fields (I believe this is where I’m stuck). How do I access addFile() of plupload for a specific GF upload field? Or is there another way to turn a url into a file upload while keeping the option for the user to manually upload a file?)

function getImageFromUrl(URL) {
		var xhr = new mOxie.XMLHttpRequest();'GET', URL);
		xhr.responseType = 'blob';
		xhr.onload = function() {
	function addToGFUploader(file) {
//how do I access the plupload for #gform_multifile_upload_4_5? Is there a hook I can use?
	  var uploader = $('#gform_multifile_upload_4_5').plupload('getUploader'); 

I think this might do the trick:

function addToGFUploader(file) {

That seems to add the file properly and it shows up in the upload folder. Might not be the cleanest or nicest way to do it but it appears to work so far.