I'm not sure where you are calling the web service to initiate this process.
Are you saying you want to open an InfoPath form and pass the userName
property to a web service within infopath which would open another infopath
form? If that is the case, I'm not sure if you can do it.
One option to explore is to have your web service return a blank form for
the desired InfoPath form template. The Xml you are returning has the
embedded template and content type information in the document so when you
open it, the correct infopath form would be opened. You can do this via an
aspx page which contains a link. In the aspx page the form xml is retrieved
and returned as the http response. When IE sees the content type (which you
have to set to: "application/ms-infopath.xml"), it will try to open infopath
with the xml in the response from the server.