Question about Access-Challenge

Hello, I've got Mobile OTP to work with FreeRadius, I'd like to take this one step further and turn this into a two phase process. The objective is to first take the pin, authenticate that and then communicate to the NAS with a challenge to receive the OTP from the user. I think this can be done via an access-challenge reply to the NAS. My question is how do I get FreeNAS to send an Access-Challenge once it has verified the PIN is correct? If anyone can kindly give me some hints or point me in the right direction. Thank you, JJ Abdi