Extending ci_sessions table

[eluser]thomasR[/eluser]
I'm currently trying to create a list of 'active users' meaning users that are logged in and have been active in the last few minutes.

Since the session stores the 'last_activity' field in the ci_sessions table I hope to use that.
My problem is, that I can't connect the sessions with the users in an SQL statement since all userdata is stored in a single text field.

Is it possible to extend the ci_sessions table to add a user_id field? Or would there be another way of doing it?