function sendAdditionalData(form, params, responseHandler)
{
if (typeof(form)=='string') form=document.getElementById(form);
var formData = new FormData(form);
if (params!=null)
{
for (paramName in params)
{
formData.append(paramName, params[paramName]);
}
}
ну и тут собсно швыряете данные куда надо
makeRequest(form.action, formData, responseHandler);
}