Short answer: No.
Long(ish) answer: Yes, but it would be an immense headache. You would have to find the session files on the hard disk, try to open them, decode them and read the userID.
I suppose you have a mysql table somewhere that holds your users? If not, create one and make sure it has a column with a timestamp which gets updated to NOW() on each pageview. Then, to show online users just show from that table users that have the timestamp set to NOW() - 5 minutes or more.
If you need any help on that, let us know.