I suggest you to play with the app.ZipFolder(folderPathh, pathToSaveZip);
//If you want the zip file conversion no accessible, you can save it in the app folder.
var fld = app.GetPrivateFolder("zipped");
var path = "/sdcard/MyFolder";
var pathName = path.substring(path.lastIndexOf("/")+1,path.length);
if(!app.FolderExists(fld))
app.MakeFolder(fld);
app.ZipFolder(path, fld+"/"+pathName);