Ah ok; that's much clearer information. I checked on those specific gateways, and they appear to be versatile enough to do it several ways. The easiest way would be to give each gateway a static IP address, disable registration, and use user/password authentication to select the proper outbound channel, and set your Kamailio server IP as the outbound server address. Then setup the "trunks" as regular outbound carriers in Kazoo, with user/pass, and add the gateway's IP to the carrier ACLs on your Kamailio servers to allow incoming calls.
You may also be able to mess with some settings in the gateways and avoid using individual credentials for each line, and use a single credential for each gateway, and have a ring group or something within the gateway select a free outbound line.