freeradius + Google Authentication (OTP) - Challenge-Response

Hello, I have a RADIUS with OTP working correctly, expcept that i want to separate the password (of LDAP user) of the OTP code generate by G.A. It seems that the solution is to generate a Challenge-Response in freeradius and SSH for SSH it's ok but for freeradius i don't see where i can activate him. Any idea ? Thanks !