How do I pass the intranet user ID as a parameter for a XML data source URL for a Value Set?

I have a how-to question I hope to get answered.   Question, how do I pass in the intranet user ID (e.g. johndoe@us.ibm.com) of the work item creator as a parameter for a Value Set?   Thus, if "John Doe" is the creator of a work item, I want to construct the URL as http://myXMLDataWebService/johndoe@us.ibm.com    Is this possible, if so, what built-in attribute of the work item do I use AND what is the syntax for passing this into the XML data source URL for a Value Set?  I am using the RTC 4.0.4 Windows client.