I am already have some practice to integrate Kamailio with Asterisk, when all users creates and registers in Kamailio, and calls go to/from Asterisk with static "host=kamailio_ip" settings for each user on Asterisk side.
I can't (don't know - how to) use in same way integration with FreeSWITCH.
Can't create in FS directory structure a user with "host=kamailio_ip", FS require registration.
Maybe I can register user on Kamailio and send additional registration request to FS with src ip changed to kamailio (lan ip)?