Is there any way to dynamically add gallery items in Magnific Popup that is already open? (or update the current item). Can't find anything about in in plugin docs.
Yes, it's possible:
// get instance (after popup was opened)
var mfp = $.magnificPopup.instance;
// modify the items array (push/remove/edit)
mfp.items.push({
src: "some-image.jpg"
});
// call update method to refresh counters (if required)
mfp.updateItemHTML();
Regarding to what to put in items
array, refer to documentation - http://dimsemenov.com/plugins/magnific-popup/documentation.html#from-the-items-option