I'm not sure if this is what you're looking for, but you can accomplish this with the Out of the Box "Query" WindchillConnector Service. It's probably a bit more complicated than having an InfoEngine task that gets the ufid, but it's another option.
Basically what you would do is create a Data Shape for WTUser, then in the Query service, leave the "containerUfid" blank, enter "name=<some username> " as the criteria, the type as "wt.org.WTUser", and the dataShape as your WTUser Data Shape. When you run the service, you should get an InfoTable back containing all the WTUser fields, one of which being the ufid.
Here's a sample screenshot of what I mean.
Hopefully this helps!