Hi,
Currently there is no supported way. (not to mention this is a group
for data export and not tracking)
But GA works by persisting values to represent a visitor and session.
Ideally you would want your server side downloads to be for the same
visitor, within in the same session as being tracking by the user on
your site. So you'd need a supported way to read our ga.js tracking
cookies and then reconstitute that data on your server to send to GA.
Both steps are currently not available.
But this is a great use case.
-Nick