There is no constructor to create document from base64 in AEM forms OSGihttps://helpx.adobe.com/experience-manager/6-5/forms/javadocs/index.html?com/adobe/aemfd/docmanager/class-use/Document.html
If you just want to store the data as an xml file you can configure the form to submit to an AEM workflow otherwise you will have to write some custom code to do the same