Found a rough solution:
var new_id = new_id || '';
var orig_new_id = new_id;
var rx = /"id":(\d+)/g;
var rx2 = /"_parentId":(\d+)/g;
var node = $('#tg').treegrid('getSelected');
var node_json = JSON.stringify(node);
node_json2 = node_json.replace(rx,function(){new_id = new_id+' '; return "\"id\":"+"\""+new_id+"\"";});
new_id = orig_new_id;
node_json2 = node_json2.replace(rx2,function(){new_id = new_id+' '; return "\"_parentId\":"+"\""+new_id+"\"";});
node = JSON.parse(node_json2);
delete node._parentId;
$('#tg').treegrid('append',{'data':[node]});
New ID could be a running number but I need it to come from database anyway.