adding an item to dhtmlxFolders

After I’ve already created a dhtmlxFolders object and loaded it via an XML string, how can I add a new item to it programatically? Can I just give it a new XML string with just one item in it and have it append to it’s list of items?

Yes , you can use loadXML or loadXMLString for such operation - additional data will be added to the end of current item-set.
Also , you can add item from js code by folder.addItem method