This is a locked functionality from Adobe. You would have to buy an additional package named "security server". It's f%#¤%¤# amazing that it's not included in the Reader Extention Server we bougth. As all other server applications from Adobe this doesn't come cheap......
Whenever you buy security server, please e-mail me to tell me your experiences!
The actual JS to make such a filename is quite simple:
var user = TextField1.rawValue;
var today = new Date();
var filename = user + "-" + today.getFullYear() + today.getMonth() + today.getDate() + ".xml";
xfa.host.exportData(filename, 0);
best regards,
Håkon