SSH PAM authentication before local authentication

Dear, finally I could implement the SSH PAM authentication of Linux users against a Freeradius server. Everything was OK, I had to create a local user in my Linux server (without password) and the authentication was against my freeradius server. The clients com efrom other boxes, both windos and linux. But now, suppose the freeradius server is offline, how can I tell the Linux server that if Freeradius is offline it has to authenticate in local mode (local PAM). In this way, I have to have a special user in my Linux server to access it through SSH in case freeradius is off. Thanks a lot, Roberto