automatic radcheck user addition - is it possible?

Hi, I've had a great success using freeradius with Coova Chilli to make a wifi hotspot and authenticate users by Mac address (so that users see a "splash" page and then click to gain Internet access). Now I'm wondering if its possible to make freeradius insert an unknown (and, therefore, normally unauthenticated) user into the radcheck/radusergroup tables IF they don't already exist in there? I'm interested in doing this so that I can transparently use freeradius to monitor Internet usage (by mac address again) without needing them to use my web ["splash"] pages at all (so that web access isn't necessary). thanks for any help! Derek