Can I run a shell command after an authentication session?

Dear all, I have a freeradius server on Debian3.1. And I have a shell command to execute after an authentication session. In fact, I want to limit the time of user connection. I know that it's not freeradius' job. So I want to write a shell command for private use. I just wonder if I can communicate with freeradius with a dependent script. Thanks for your attention and look forward to your reply. Sincerely, Kun